Electrolyte solution and method for producing same, continuously dissolving facility, electrolyte membrane, electrode catalyst layer, membrane electrode assembly and fuel cell

ABSTRACT

A method for producing an electrolyte solution including a supply step of continuously supplying an emulsion based a polymer electrolyte and a solvent into a dissolution facility, and a dissolution step of continuously dissolving the polymer electrolyte in the solvent by heating the interior of the dissolution facility to obtain the electrolyte solution.

TECHNICAL FIELD

The present invention relates to an electrolyte solution and a methodfor producing the electrolyte solution, a continuously dissolvingfacility, an electrolyte membrane, an electrode catalyst layer, amembrane electrode assembly and a fuel cell.

BACKGROUND ART

Recently, there has been a growing demand for solid polymer electrolytefuel cells. In producing e.g., membranes and electrodes for solidpolymer electrolyte fuel cells, a solution of fluorine-based polymerelectrolytes (hereinafter referred to also as “fluorine-based polymerelectrolyte”) having a sulfonate functional group (hereinafter referredto also as “H-type”) is used.

It is required that these electrolyte membranes and electrodes have hotwater dissolution resistance in order to improve output characteristicsof fuel cells. It is also required that a solution having an electrolytehighly dispersed therein is produced in a short time. In addition, as amaterial for an electrolyte membrane and an electrode a highlyconcentrated electrolyte solution is preferable in view of handling.

In conventional methods for producing a fluorine-based polymerelectrolyte solution, a fluorine-based polymer electrolyte is generallydissolved in a water/alcohol mixed solvent while stirring in abatch-type closed reactor, such as an autoclave electrolyte under hightemperature and pressure.

For example, Patent Literature 1 discloses a method of dissolving a bulkof a perfluorosulfonated polymer electrolyte in a water/ethanol mixedsolvent at 165° C. for 7 hours to solid content of approximately 5 mass% solution, in an autoclave made of SUS304 having a glass innercylinder.

Patent Literature 2 discloses a method of supplying a micron-orderfine-particles dispersion without clogging by controlling the angle of aflow channel.

Patent Literature 3 discloses a method of treating anelectrolyte-containing solution with heat at a temperature of the glasstransition temperature of the electrolyte to 300° C.

Patent Literature 4 discloses a method of suspending organic andinorganic components in water, bringing the water into a near-criticalor supercritical state and passing the aqueous solution of this statethrough a tubular reactor.

Typical examples of the H-type fluorine-based polymer electrolytesolution include Nafion <registered trade mark> Dispersion Solution(manufactured by DuPont in the United States) and Aciplex <registeredtrade mark> —SS (manufactured by Asahi Kasei E-materials Corporation).However, since the solubility of the H-type fluorine-based polymerelectrolyte to a solvent is extremely low, various techniques have beenso far proposed for use in a method for producing an electrolytesolution.

For example, Patent Literature 5 discloses methods of dissolving bulksof H-type and sodium-type (hereinafter referred to also as “Na-type”)fluorine-based polymer electrolytes in a solvent containing water or awater-immiscible organic solvent at a high temperature of 200° C. ormore.

Patent Literature 6 discloses a method of dissolving a bulk of a Na-typefluorine-based polymer electrolyte in water at a high temperature of200° C. or more. Patent Literature 7 describes a method of dissolving anemulsion of a fluorine-based polymer electrolyte by heating whilestirring in an autoclave at a temperature of 50 to 250° C. for 1 to 12hours.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent Laid-Open No. 2005-82748-   Patent Literature 2: Japanese Patent Laid-Open No. 2005-319409-   Patent Literature 3: Japanese Patent Laid-Open No. 2013-51051-   Patent Literature 4: Japanese Patent Laid-Open No. 2002-210349-   Patent Literature 5: National Publication of International Patent    Application No. 2001-504872-   Patent Literature 6: WO2009-125695A1-   Patent Literature 7: WO2011-034179A1

SUMMARY OF INVENTION Technical Problem

In Patent Literature 1, however, in dissolving a bulk of aperfluorosulfonated polymer electrolyte using an autoclave (batchsystem), it takes a long time of 7 hours to dissolve a solid-content(approximately 5 mass %) low in concentration. It cannot be said thatsuch dissolution is effective, and productivity thereof is significantlylow.

Patent Literature 2 discloses a method of supplying a fine-particledispersion solution and does not disclose continuous dissolution of apolymer electrolyte of a fine-particle dispersion solution, i.e., anemulsion.

Patent Literature 3 discloses a method for dispersing an electrolytesolution and does not disclose a continuous dissolution of a polymerelectrolyte of an emulsion. Patent Literature 4 does not disclose amethod of dissolving an electrolyte-based emulsion.

In Patent Literatures 5 and 6, the electrolyte solution obtained bydissolving a bulk of Na-type fluorine-based polymer electrolyte isextremely viscous and aggregated solution or contains a polymerelectrolyte remaining undissolved. In other words, the fluorine-basedpolymer electrolyte is insufficiently dispersed in a dissolution step.Also, in Patent Literature 5, the dissolution temperature of an H-typefluorine-based polymer electrolyte bulk is near the thermaldecomposition initiation temperature of the electrolyte, which suggeststhat thermal decomposition of the electrolyte occurs during adissolution step. In other words, it is suggested that the concentrationof a fluorine ion in the electrolyte solution is high, and that the hotwater dissolution resistance of the electrolyte membrane and electrodecatalyst layer obtained from the electrolyte solution is low. What isspecifically disclosed in Patent Literature 7 is a method of forming amembrane by directly casting an emulsion of an H-type fluorine-basedpolymer electrolyte, or a method for forming an electrode binder bydirectly mixing an emulsion of an H-type fluorine-based polymerelectrolyte with a catalyst. If the emulsion is dissolved at a hightemperature, thermal decomposition of the electrolyte takes place, withthe result that the concentration of a fluorine ion in the electrolytesolution increases and the hot water dissolution resistance of theelectrolyte membrane or the electrode catalyst layer obtained from theelectrolyte solution becomes presumably low.

The present invention was attained in view of the aforementionedproblems. An object of the present invention is to provide anelectrolyte solution production method which enables to produce anelectrolyte solution having a polymer electrolyte highly dispersedherein, efficiently with good productivity (continuously), i.e., in ashort time and in a large amount, and provide a continuously dissolvingfacility.

Another object of the present invention is to provide an electrolytesolution having a polymer electrolyte highly dispersed therein andproviding an electrolyte membrane and an electrode catalyst layer havinghigh hot water dissolution resistance.

Another object of the present invention is to provide an electrolytemembrane, an electrode catalyst layer, a membrane electrode assembly anda fuel cell with satisfactory output characteristics by using theaforementioned electrolyte solution.

Solution to Problem

The present inventors have intensively conducted studies with a view toattaining aforementioned objects. As a result, they have found that theobjects can be attained by production methods having predeterminedconstitutions, and achieved the present invention.

The present invention is more specifically as follows.

[1]

A method for producing an electrolyte solution, comprising:

a supply step of continuously supplying an emulsion comprising a polymerelectrolyte and a solvent into a dissolution facility; and

a dissolution step of continuously dissolving the polymer electrolyte inthe solvent by heating an interior of the dissolution facility to obtainthe electrolyte solution.

[2]

The method for producing the electrolyte solution according to Item [1],wherein, in the dissolution step, a heating temperature of the interiorof the dissolution facility is 150 to 350° C.

[3]

The method for producing the electrolyte solution according to Item [1]or [2], wherein, in the dissolution step, the heating temperature of theinterior of the dissolution facility is 150 to 290° C.

[4]

The method for producing the electrolyte solution according to any oneof Items [1] to [3], wherein, in the dissolution step, a pressure withinthe dissolution facility exceeds vapor pressure of the solvent at theheating temperature of the dissolution facility.

[5]

The method for producing the electrolyte solution according to any oneof Items [1] to [4], wherein, in the dissolution step, the pressurewithin the dissolution facility is controlled by use of a back pressureregulating valve so as to exceed the vapor pressure of the solvent atthe heating temperature of the dissolution facility.

[6]

The method for producing the electrolyte solution according to any oneof Items [1] to [5], further comprising, after the dissolution step, acooling step of cooling the electrolyte solution while maintaining apressure exceeding the vapor pressure of the solvent at the heatingtemperature of the interior of the dissolution facility.

[7]

The method for producing the electrolyte solution according to any oneof Items [1] to [6], wherein the dissolution facility is a tube.

[8]

The method for producing the electrolyte solution according to any oneof Items [1] to [7], wherein the polymer electrolyte contains afluorine-based polymer electrolyte.

[9]

The method for producing the electrolyte solution according to Item [8],wherein

the fluorine-based polymer electrolyte has an average particle diameterof 10 nm or more and less than 500 nm, and

the fluorine-based polymer electrolyte contains a —SO₃X group where X isan alkali metal, an alkaline-earth metal or NR₁R₂R₃R₄ where R₁, R₂, R₃and R₄ are each independently an alkyl group having 1 to 3 carbon atomsor hydrogen.

[10]

A continuously dissolving facility comprising:

a pump for continuously supplying an emulsion comprising a polymerelectrolyte and a solvent into a dissolution facility;

the dissolution facility for continuously dissolving the polymerelectrolyte in the solvent; and

heating means which heats the dissolution facility.

[11]

The continuously dissolving facility according to Item [10], wherein thedissolution facility is a tube.

[12]

An electrolyte solution obtained by the method for producing theelectrolyte solution according to any one of Items [1] to [9] orproduced by the continuously dissolving facility according to Item [10]or [11].

[13]

An electrolyte solution comprising: a fluorine-based polymer electrolytewhich contains a —SO₃X group where X is hydrogen, an alkali metal, analkaline-earth metal or NR₁R₂R₃R₄ where R₁, R₂, R₃ and R₄ are eachindependently an alkyl group having 1 to 3 carbon atoms or hydrogen; anda water-containing solvent, wherein

in a dynamic light scattering particle-size measurement, at least oneparticle-size peak top (A) is present in a range of 0.10 μm or more andless than 5.0 μm and at least one particle-size peak top (B) is presentin a range of 5.0 μm or more and 50.0 μm or less,

a scattering intensity ratio (A/B) of the particle-size peak top (A) tothe particle-size peak top (B) is 1.0×10⁻² or more and 1.0×10 or less,and

a fluorine ion concentration is 500 ppm or less based on a solid-contentmass of the fluorine-based polymer electrolyte.

[14]

The electrolyte solution according to Item [13], wherein no scatteringpeak is present in the laser diffraction/scattering particle sizedistribution measurement.

[15]

The electrolyte solution according to Item [13] or [14], wherein thefluorine-based polymer electrolyte contains a copolymer having arepeating unit represented by the following formula (1) and a repeatingunit represented by the following formula (2):—(CFZ—CF₂)—  (1)where Z represents H, Cl, F or a perfluoroalkyl group having 1 to 3carbon atoms,—(CF₂—CF(—O—(CF₂CF(CF₃)O)_(n)—(CF₂)_(m)—SO₃X))—  (2)where X is hydrogen, an alkali metal, an alkaline-earth metal orNR₁R₂R₃R₄ where R₁, R₂, R₃ and R₄ are each independently an alkyl grouphaving 1 to 3 carbon atoms or hydrogen; m is an integer of 0 to 12; andn is an integer of 0 to 2, with the proviso that m and n are notsimultaneously 0.[16]

The electrolyte solution according to Item [15], wherein

-   the Z is F,-   X is K, Na or Li,-   n is 0 and-   m is 2.    [17]

The electrolyte solution according to Item [15] or [16], wherein

-   the Z is F,-   X is Na,-   n is 0 and-   m is 2.    [18]

The electrolyte solution according to any one of Items [13] to [17],wherein the fluorine-based polymer electrolyte has an equivalent mass of300 to 1,000 g/eq.

[19]

The electrolyte solution according to any one of Items [13] to [18],wherein the fluorine-based polymer electrolyte has a solid-content of 11to 50 mass %.

[20]

The electrolyte solution according to any one of Items [13] to [19],wherein the water-containing solvent contains 80 to 100 mass % of waterand 0 to 20 mass % of an alcohol.

[21] An electrolyte solution comprising a fluorine-based polymerelectrolyte, wherein

40 mass % or more of polymer chain terminals of the fluorine-basedpolymer electrolyte is —CF₂H,

a fluorine ion concentration (mass %) is 0.10 to 500 ppm based on asolid-content mass of the fluorine-based polymer electrolyte, and

an Fe concentration is 0.01 to 10 ppm based on a solid-content mass ofthe fluorine-based polymer electrolyte.

[22]

An electrolyte membrane comprising a fluorine-based polymer electrolyte,wherein

40 mass % or more of polymer chain terminals of the fluorine-basedpolymer electrolyte is —CF₂H,

a fluorine ion concentration (mass %) is 0.10 to 500 ppm based on asolid-content mass of the fluorine-based polymer electrolyte, and

an Fe concentration (mass %) is 0.01 to 10 ppm based on a solid-contentmass of the fluorine-based polymer electrolyte.

[23]

An electrolyte membrane formed of the electrolyte solution according toany one of Items [12] to [21].

[24]

An electrode catalyst layer formed of the electrolyte solution accordingto any one of Items [12] to [21].

[25]

A membrane electrode assembly having the electrolyte membrane accordingto Item [22] or [23] and the electrode catalyst layer according to Item[24].

[26]

A fuel cell having the membrane electrode assembly according to Item[25].

Advantageous Effects of Invention

According to the present invention, it is possible to provide anelectrolyte solution production method which enables to produce anelectrolyte solution having a polymer electrolyte highly dispersedtherein, efficiently with good productivity, i.e., in a short time andin a large amount, and provide a continuously dissolving facility.

According to the present invention, it is possible to further provide anelectrolyte solution having a polymer electrolyte highly dispersedtherein and providing an electrolyte membrane and an electrode catalystlayer having high hot water dissolution resistance.

According to the present invention, it is possible to further provide anelectrolyte membrane, an electrode catalyst layer, a membrane electrodeassembly and a fuel cell with satisfactory output characteristics byusing the aforementioned electrolyte solution.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a schematic view showing a continuously dissolving facilityaccording to an embodiment of the invention.

FIG. 2 shows a graph showing the particle distributions offluorine-based polymer electrolytes in the electrolyte solutionsaccording to Examples and Comparative Examples.

DESCRIPTION OF EMBODIMENTS

Now, the embodiment for carrying out the invention (hereinafter referredto as the “the present embodiment”) will be more specifically describedbelow; however, the present invention is not limited to the presentembodiment and can be modified in various ways without departing fromthe spirit of the invention.

[Method for Producing an Electrolyte Solution]

The method for producing an electrolyte solution according to thepresent embodiment includes a supply step of continuously supplying anemulsion comprising a polymer electrolyte and a solvent into adissolution facility and a dissolution step of continuously dissolvingthe polymer electrolyte in the solvent by heating the interior of thedissolution facility to obtain an electrolyte solution.

If the method for producing an electrolyte solution as mentioned aboveis employed, dispersibility of a fluorine-based polymer electrolyte inan electrolyte solution is more improved, and an electrolyte solutionimproved in dispersibility can be obtained in a shorter time even if ahighly concentrated polymer electrolyte (high solid-contentconcentration) is used. Particularly, if the above steps arecontinuously carried out, an electrolyte solution can be moreefficiently produced in a short period of time in a large amount. Now,each of the steps will be more specifically described below.

[Supply Step]

The supply step is a step of continuously supplying an emulsioncomprising a polymer electrolyte and a solvent into a dissolutionfacility. Examples of the supplying method, although it is notparticularly limited as long as it can feed an emulsion, include afeeding method by a pump.

Note that, in the specification of the present application,“continuously” means that an operation is not carried out in a batchsystem. The case where the time zone for supplying raw materials and thetime zone for discharging e.g., a product and a reaction solution are atleast partly overlapped; the case where raw materials and the like arecontinuously supplied and e.g., a product and a reaction solution arecontinuously discharged; and the case where materials to be treated arecontinuously transferred even intermittently, are included.

[Emulsion]

The emulsion is a solution prepared by dispersing particles of a polymerelectrolyte in a solvent. Such an emulsion can be produced by a methoddescribed, for example, in WO2011-034179A1; however the method is notparticularly limited to this. It is preferable that operations such ascoagulation and drying of emulsion particles are not included during aprocess for producing an emulsion. If particles are maintained withoutany coagulation, the emulsion in which particles having an averageparticle diameter of 10 or more and less than 500 nm are dispersed canbe easily obtained.

As an emulsion, an emulsion in which particles of a polymer electrolyteis dispersed in a solvent with the help of an emulsifier is acceptable.An emulsion may be formed of a single type of polymer electrolyte or twoor more types of polymer electrolytes in combination. Other additivesmay be added to an emulsion.

[Polymer Electrolyte]

The polymer electrolyte is not limited as long as it can form anemulsion, and examples thereof include a polymer electrolyte comprisingan —SO₃X group, a —COOX group or a —PO₃X₂ group (X is hydrogen, analkali metal, an alkaline-earth metal or NR₁R₂R₃R₄; R₁, R₉, R₃ and R₄are each independently an alkyl group having 1 to 3 carbon atoms orhydrogen); a polymer electrolyte that can form an emulsion with the helpof an emulsifier; a polymer electrolyte not comprising an —SO₃X group, a—COOX group or a —PO₃X₂ group and capable of forming an emulsion orslurry without the help of an emulsifier, or other polymer electrolytecapable of being dispersed in solvents. Among them, a polymerelectrolyte comprising an —SO₃X group, a —COOX group or a —PO₃X₂ groupis preferable. If such a polymer electrolyte is used, dispersibility ofthe polymer electrolyte after dissolution tends to be more improved.

Note that the term “electrolytes” used in the present inventiongenerally indistinguishably includes a precursor (ended with e.g.,—SO₂F) thereof.

(Fluorine-Based Polymer Electrolyte)

The polymer electrolyte is preferably a fluorine-based polymerelectrolyte, more preferably a fluorine-based polymer electrolyte havingan —SO₃X group, a —COOX group or a —PO₃X₂ group and further preferably afluorine-based polymer electrolyte having an —SO₃X group. If such apolymer electrolyte is used, the solubility of the polymer electrolytein a solvent tends to be further improved.

The fluorine-based polymer electrolyte is not particularly limited. Forexample, a fluorine-based polymer electrolyte containing a copolymerhaving a repeating unit represented by the following formula (1) and arepeating unit represented by the following formula (2) is preferable.—(CFZ—CF₂)—  (1)

(in the above formula (1), Z represents H, Cl, F or a perfluoroalkylgroup having 1 to 3 carbon atoms)—(CF₂—CF(—O—(CF₂CF(CF₃)O)_(n)—(CF₂)_(m)—SO₃X))—  (2)

(in the above formula (2), X is hydrogen, an alkali metal, analkaline-earth metal, or NR₁R₂R₃R₄ where R₁, R₂, R₃ and R₄ are eachindependently an alkyl group having 1 to 3 carbon atoms or hydrogen, mis an integer of 0 to 12 and n is an integer of 0 to 2; with the provisothat m and n are not simultaneously 0).

Among them, a fluorine-based polymer electrolyte in which Z is F; X isK, Na, or Li; n is 0; and m is 2, is preferable. Furthermore, afluorine-based polymer electrolyte in which Z is F; X is Na; n is 0; andm is 2, is more preferable. If such a fluorine-based polymer electrolyteis used, an electrolyte solution having a higher dispersibility can beobtained in a shorter time.

The fluorine-based polymer electrolyte may have other functional groups.Examples of the other functional groups include, but are notparticularly limited to, an —SO₂F group, a —CF₂H group and a —CF₃ group.

The average particle diameter of a polymer electrolyte in an emulsion,which is determined by dynamic light scattering particle-sizemeasurement, is preferably 10 nm or more and less than 500 nm, morepreferably 50 nm or more and 300 nm or less and further preferably 100nm or more and 200 nm or less. If the average particle diameter of apolymer electrolyte falls within the above range, the stability of theparticles of the polymer electrolyte is improved and the particles ofthe polymer electrolyte can be easily produced. Note that the averageparticle diameter can be determined by the dynamic light scatteringparticle-size measurement described in Examples.

(Equivalent Mass)

The equivalent mass of a polymer electrolyte in an emulsion ispreferably 300 to 1,000 g/eq, more preferably 400 to 900 g/eq andfurther preferably 500 to 800 g/eq. If the equivalent mass is 300 g/eqor more, e.g., an electrolyte membrane having further excellent powergeneration performance tends to be obtained. In contrast, if theequivalent mass is 1,000 g/eq or less, e.g., an electrolyte membranehaving lower water-absorbing property and more excellent mechanicalstrength tends to be obtained. The “equivalent mass of a polymerelectrolyte” herein refers to a dry mass per equivalent of a sulfonategroup. Note that equivalent mass of a polymer electrolyte can bemeasured by the method described in Examples (described later).

(Solid-Content Concentration)

The solid-content concentration of a polymer electrolyte in an emulsionis preferably 11 to 50 mass %, more preferably 15 to 45 mass % andfurther preferably 20 to 40 mass %. If the solid-content of a polymerelectrolyte is 11 mass % or more, the yield per unit time tends tobecome more excellent. In contrast, if the solid-content of a polymerelectrolyte is 50 mass % or less, difficulty of handling due togeneration of undissolved matter and an increase of viscosity tend to bemore suppressed. The solid-content concentration can be measured by themethod described in Examples.

(Melt Flow Rate)

The melt flow rate (MFR) of a polymer electrolyte in an emulsion ispreferably 100 g/10 minutes or less, more preferably 10 g/10 minutes orless and further preferably 5 g/10 minutes or less. If MFR is 100 g/10minutes or less, the output characteristics of a fuel cell tend to bemaintained for a longer time. In contrast, MFR is preferably 0.01 g/10minutes or more, more preferably 0.1 g/10 minutes or more and furtherpreferably 0.3 g/10 minutes or more. If MFR is 0.01 g/10 minutes ormore, a polymer electrolyte tends to successfully and more efficientlydissolved. Note that MFR can be measured by the method described inExamples.

(Spherical Shape)

The particles of a polymer electrolyte in an emulsion are preferablyspherical. The “spherical” herein refers to a shape having an aspectratio of 3.0 or less. Generally, as the aspect ratio of a shape comescloser to 1.0, the shape becomes closer to a sphere. The aspect ratio ofthe spherical particles is preferably 3.0 or less, more preferably 2.0and further preferably 1.5. The lower limit of the aspect ratio of thespherical particles is preferably 1.0. If the aspect ratio falls withinthe above range, the viscosity of the electrolyte solution furtherdecreases and handling tends to be improved even if the solid-contentmass of a polymer electrolyte is increased. Note that the aspect ratiocan be measured by the method described in Examples.

[Solvent]

Examples of the solvent include, but not particularly limited to, waterand a water-organic solvent mixture. Examples of such an organic solventinclude, but not particularly limited to, protic organic solvents suchas an alcohol and glycerin; and aprotic solvents such asN,N-dimethylformamide, N,N-dimethylacetamide and N-methylpyrrolidone.These can be used alone or in combination of two or more.

Preferable examples of the alcohol include, but not particularly limitedto, a low boiling-point alcohol having 1 to 3 carbon atoms. Thesealcohols may be used alone or in combination of two or more. Examples ofthe low boiling-point alcohol having 1 to 3 carbon atoms include, butnot particularly limited to, at least one alcohol selected from thegroup consisting of methanol, ethanol, 1-propanol and 2-propanol. Amongthem, ethanol and 1-propanol are preferable. If such an alcohol is used,the dispersibility of a polymer electrolyte in an electrolyte solutiontends to be more improved and the affinity of the polymer electrolytetends to be more improved.

The content of water in a solvent is preferably 80 to 100 mass %, morepreferably 90 to 100 mass % or more and further preferably 100 mass %.If the concentration of water is the above concentration, thedispersibility of a polymer electrolyte in an electrolyte solution tendsto be more improved. Because of this, an electrolyte solution comprisinga polymer electrolyte having a higher solid-content mass tends to besuccessfully obtained.

The content of an organic solvent in the solvent is preferably 0 to 50mass %, more preferably 0 to 20 mass %, further preferably 0 to 10 mass% and still further preferably 0 mass %. If the concentration of anorganic solvent is the above concentration, the dispersibility of apolymer electrolyte in an electrolyte solution tends to be moreimproved. Because of this, an electrolyte solution comprising a polymerelectrolyte having a higher solid-content mass tends to be successfullyobtained. Furthermore, using no flammable organic solvent is preferablein a safety point of view.

(Transmittance in UV Measurement)

As a reference for estimating degree of dissolution, a transmittance ofa solution having a solid-content of 20 mass % at a wavelength of 800 nmin UV measurement can be used. The transmittance of an emulsion ispreferably less than 90% T, more preferably less than 70% T and furtherpreferably less than 50% T. If the transmittance is less than 90% T, theparticles constituting an emulsion are small in size and tend to be moreeasily dissolved.

[Dissolution Step]

The dissolution step is a step of continuously dissolving a polymerelectrolyte in a solvent by heating the interior of a dissolutionfacility to obtain an electrolyte solution. If such a dissolution stepis employed, an electrolyte solution can be obtained in a shorter periodof time.

Note that the “dissolution” refers to a treatment of dissolving orfinely dispersing a polymer electrolyte of an emulsion in a solvent toobtain a polymer solution. Whether a polymer electrolyte is dissolved ornot can be determined and confirmed based on scattering-intensity ratioin dynamic light scattering particle-size measurement (described later)or/and transmittance in UV measurement.

In the dissolution step, the heating temperature within the dissolutionfacility is preferably 50° C. or more, more preferably 100° C. or more,further preferably 150° C. or more, still further preferably 200° C. ormore and further more preferably 250° C. or more. In contrast, in thedissolution step, the heating temperature within the dissolutionfacility is preferably 500° C. or less, more preferably 400° C. or less,further preferably 350° C. or less, still further preferably 300° C. orless and further more preferably 290° C. or less. If the heatingtemperature is 50° C. or more, the solubility and dispersibility of apolymer electrolyte in a solvent tend to further improve. In contrast,if the heating temperature is 500° C. or less, the thermal decompositionof a polymer electrolyte tends to be further suppressed. Note that, theheating temperature within the dissolution facility refers to, forexample, the temperature of a thermostatic bath or the like or the valueof temperature of the emulsion within the dissolution facility actuallymeasured, or refers to both of the temperatures. With respect to theactual temperature of the emulsion or electrolyte solution within thedissolution facility, the temperature of at least a part of the flowchannel of the dissolution facility is preferably equivalent to theheating temperature within the dissolution facility, in view of thedispersibility of the electrolyte solution.

In the dissolution step, the dissolution time (residence time of anemulsion in the dissolution facility) is not particularly limited, sincemore preferable range of the dissolution time varies depending upon thedissolution method. The dissolution time is preferably one minute ormore, more preferably two minutes or more and further preferably threeminutes or more. In contrast, the dissolution time is preferably 240minutes or less, more preferably 120 minutes or less, further preferably90 minutes or less and still further preferably 60 minutes or less. Ifthe dissolution time falls within the above range, the dispersibility ofa polymer electrolyte is more improved and the thermal decomposition ofthe polymer electrolyte tends to be more suppressed. Note that theresidence time of an emulsion in a dissolution facility is representedby the value obtained by dividing the content volume in the heateddissolution facility by the feed rate of a pump.

The pressure within a dissolution facility preferably exceeds the vaporpressure of a solvent at the heating temperature of the dissolutionfacility, and is preferably not more than the preset pressure of asafety valve provided in the facility. If the pressure within thedissolution facility falls within the above range, the solubility of apolymer electrolyte in a solvent is further improved and a solution inwhich such a polymer electrolyte is dissolved in a higher level tends tobe obtained in a shorter time. In addition, the solution can be fedwithout causing clogging in the dissolution facility. Note that thepressure within a dissolution facility is preferably controlled by useof a back pressure regulating valve so as to exceed the vapor pressureof the solvent at the heating temperature of the dissolution facility.

Note that in order to improve the dissolution efficiency, thedissolution facility is preferably a tube. In order to control pressure,the above dissolution step is preferably performed in a closed reactor.The details of the dissolution facility will be described later.

[Cooling step]

The method for producing an electrolyte solution of the presentembodiment may further have, after the dissolution step, a cooling stepof cooling the electrolyte solution while maintaining a pressure beyondthe vapor pressure of a solvent at the heating temperature within thedissolution facility. If such a cooling step is employed, theelectrolyte solution can be fed outside the dissolution facility withoutclogging of the dissolution facility. After the cooling step, thepressure is reduced. In this manner, the electrolyte solution can beobtained.

[Discharge Step]

The method for producing an electrolyte solution of the presentembodiment may have a discharge step for discharging the electrolytesolution out of the dissolution facility. Examples of the dischargemethod include, but not particularly limited to, a method fordischarging an electrolyte solution through a back pressure regulatingvalve (described later).

[Ion Exchange Step]

The method for producing an electrolyte solution of the presentembodiment may further have, after the dissolution step, a cooling stepor a discharge step, an ion exchange step of ion exchanging to H in thecase where X is an alkali metal or an alkaline-earth metal. If an ionexchange step is employed, power generation performance of a fuel cellprepared by using an electrolyte membrane or the electrode catalystlayer using the electrolyte solution tends to be improved. Note thatexamples of a method for exchanging ions, include, but not particularlylimited to, a method of passing an electrolyte solution through acationic exchange resin.

[Continuously Dissolving Facility]

The continuously dissolving facility of the present embodiment has apump for continuously supplying an emulsion comprising a polymerelectrolyte and a solvent into a dissolution facility; a dissolutionfacility for continuously dissolving the polymer electrolyte in thesolvent; and a heating mean which heats the dissolution facility. FIG. 1shows a schematic view of the continuously dissolving facility of thepresent embodiment.

[Pump]

The pump is used for continuously supplying an emulsion comprising apolymer electrolyte and a solvent to a dissolution facility. The pumpmay be provided downstream or upstream, in a supply direction, of thedissolution facility, both upstream and downstream thereof or within thedissolution facility.

Examples of the types of pump include, but are not particularly limitedto, a turbo pump, a piston pump, a plunger pump, a diaphragm pump, agear pump, a vane pump and a screw pump. Among them, a pump having anexcellent pressure resistance in view of safety and high quantitativeperformance and a high discharge pressure in view of productivity, suchas a plunger pump and a diaphragm pump, is preferable. To suppresspulsation, use of a multiple pump or an accumulator is more preferable.

[Dissolution Facility]

The dissolution facility is used for continuously dissolving a polymerelectrolyte in a solvent. The dissolution facility is heated by theheating means later described. A polymer electrolyte in an emulsion isdissolved in a solvent when the emulsion passes through the dissolutionfacility depending upon the conditions within the dissolution facilityand a homogeneous electrolyte solution is discharged from thedissolution facility.

The ratio of the heat-transfer area (inner area) of a dissolutionfacility to the volume of an emulsion within the dissolution facility ispreferably 40 to 40000, more preferably 80 to 8000 and furtherpreferably 400 to 2000. If the ratio of the heat-transfer area (innerarea) of a dissolution facility to the volume of an emulsion within thedissolution facility falls within the above range, the dissolutionefficiency tends to be improved.

Examples of the dissolution facility include, but not particularlylimited to, a dissolution facility in which a fluid flowing within thefacility follows a plug flow model.

The dissolution facility, although it is not particularly limited, ispreferably a tube formed of a metal. As the material for the dissolutionfacility, a suitable material in view of corrosion resistance may beselected. Examples thereof include a SUS-based material, aHastelloy-based material, a titanium-based material, a zirconia-basedmaterial and a tantalum-based material. Among them, in consideration ofbalance between corrosion resistance and cost, a SUS-based material anda material having the same composition as in Hastelloy (registered trademark of Haynes International, Inc. in the United States) are preferable.Of them, SUS316 and a material having the same composition as inHastelloy C are preferable. Of them, a material having the samecomposition as in Hastelloy C276 is preferable. Note that the samecompositions as in Hastelloy, Hastelloy C and Hastelloy C276 refer tocompositions comprising Ni (56 to 60 mass %), Cr (16 to 22 mass %), Mo(13 to 16 mass %), W (2 to 6 mass %), Fe (3 to 8 mass %) and Co (2.5mass % or less). If a dissolution facility made of such a metal is used,the dissolution step can be performed at a relatively high temperatureand at a high pressure, and the polymer electrolyte to be contained inthe resultant electrolyte solution tends to have a polymer chain withrelatively stable ends. The inner wall of the tube may have lining.Examples of the lining, although it is not particularly limited to,include fluorine lining and glass lining. If a dissolution facilityprovided with such lining is used, the dissolution step can be performedat a relatively low temperature and at a low pressure, with the resultthat the concentrations of F ions and Fe ions of the resultantelectrolyte solution tend to be suppressed to a low level.

As the form of the dissolution facility, although it is not particularlylimited, for example, a tube form is preferable. If the dissolutionfacility has a tube form, productivity and dissolution efficiency tendto more improve. The tube form is not particularly limited. Examples ofthe tube form include a linear, coil and angular tubes. Among them, acoil tube is preferable in view of footprint and stable operation. Theouter diameter of the tube, in view of productivity and dissolutionefficiency, is preferably 1/16 to 2 inches and more preferably ¼ to ½inches. Note that, pipes of 6 A to 500 A generally available in themarket may be used. In view of productivity and dissolution efficiency,e.g., an in-line mixer, wire mesh and metal filling may be provided inthe tube.

The wall thickness of the tube, although it is not particularly limited,may be appropriately selected in view of pressure resistance. The innerdiameter of the tube is preferably 1 to 50 mm and more preferably 4 to50 mm, in view of productivity and dissolution efficiency.

The surface of the inner wall of a tube may be a rough or mirrorsurface. In view of dissolution efficiency, a maximum height of theprojections and depressions is preferably 50 μm or less, more preferably25 μm or less and further preferably 10 μm or less. The maximum heightis obtained, for example, by using a laser microscope, taking a standardlength alone from a roughness curve, in the direction of its averageline, and measuring the interval between the summit line and the valleyline of the part thus taken along the direction of the longitudinalmagnification of the roughness curve.

The length of a tube is determined depending upon the requisitedissolution time (=residence time). More specifically, the length of thetube can be calculated based on the inner diameter of the tube so that aproduct of dissolution time (min) and a feed rate (L/min) becomes equalto or more than the content volume of the heated tube.Dissolution time (min)×feed rate (L/min)≥content volume of tubeContent volume of tube=(tube inner diameter/2)²×π×tube length

When a plurality of dissolution facilities (tubes) are used in series,e.g., unions (whose shapes may be the same or different), T-type unionsfor connecting the dissolution facilities, a check valve, a safetyvalve, a back pressure regulating valve, a pressure gauge and athermometer may be provided between the dissolution facilities. Notethat, if a plurality of dissolution facilities are arranged next to eachother, thereby increasing the content volume, productivity can beimproved.

[Heating Means]

The heating means is used for heating a dissolution facility. Examplesof the heating method, although it is not particularly limited to,include a method of using e.g., a heating medium such as hot air, hotwater, steam and silicone oil for heating a dissolution facility. Amongthem, hot air is preferable since it is easy to use. A dissolutionfacility can be placed in a thermostatic bath an atmosphere of which isset at a particular temperature by hot air.

[Pressure Control Means]

It is preferable that the continuously dissolving facility of thepresent embodiment further has a pressure control means, which controlsthe pressure within a dissolution facility so as to exceed the vaporpressure of a solvent at the heating temperature of the dissolutionfacility. The pressure control means may be provided downstream orupstream, in a supply direction, of the dissolution facility, bothupstream and downstream thereof or within the dissolution facility.

Examples of the pressure control means, although it is not particularlylimited to, include a back pressure regulating valve, an automaticpressure regulating valve (PIC) and a pump (described above). If theback pressure regulating valve or an automatic pressure regulating valve(PIC) is used to thereby maintain the pressure within a dissolutionfacility to be constant, in other words, suppress pressure fluctuationas much as possible, the dispersibility of an electrolyte solution isimproved and clogging in a dissolution facility tends to be prevented.Furthermore, if the pump is used, the interior pressure of thedissolution facility can be increased.

Note that a portion from the pump to the pressure control means (backpressure regulating valve) can be regarded as a closed reactor having aconstant pressure. If such a continuously dissolving facility isemployed, the dispersibility of a fluorine-based polymer electrolyte inan electrolyte solution is more improved. In addition, such anelectrolyte solution improved in dispersibility tends to be obtained ata higher concentration and in a shorter time.

[Cooling Means]

The continuously dissolving facility of the present embodimentpreferably has a cooling means, which cools an electrolyte solutionwhile maintaining the pressure beyond vapor pressure of a solvent at theheating temperature within the dissolution facility, downstream of thedissolution facility. If such a cooling means is employed, cloggingwithin the dissolution facility caused in discharging the electrolytesolution tends to be more successfully suppressed.

Examples of the cooling method, although it is not particularly limitedto, include a method of cooling an electrolyte solution by allowing theelectrolyte solution to pass through a cooling pipe and a method ofair-cooling an electrolyte solution by allowing the electrolyte solutionto pass through a tube of room temperature without passing through thecooling pipe.

[Electrolyte Solution (First Embodiment)]

The electrolyte solution of the first embodiment can be obtained by amethod for producing an electrolyte solution as mentioned above orproduced by a continuously dissolving facility as mentioned above.

If such an electrolyte solution is used, an electrolyte solution havinga polymer electrolyte highly dispersed therein can be provided.Furthermore, if such an electrolyte solution is used, an electrolytemembrane and electrode catalyst layer having high hot water dissolutionresistance can be provided. Note that electrolyte solution of the firstembodiment may contain the contents of the electrolyte solution of thefirst embodiment described later.

[Electrolyte Solution (Second Embodiment)]

The electrolyte solution of the second embodiment contains afluorine-based polymer electrolyte having a SO₃X group (X is an alkalimetal, an alkaline-earth metal or NR₁R₂R₃R₄ where R₁, R₂, R₃ and R₄ areeach independently an alkyl group having 1 to 3 carbon atoms orhydrogen) and a water-based solvent. In the electrolyte solution of thesecond embodiment, in dynamic light scattering particle-sizemeasurement, at least one particle-size peak top (A) within the range of0.10 or more and less than 5.0 μm and at least one particle-size peaktop (B) within the range of 5.0 or more and 50.0 μm or less are present;and the scattering intensity ratio (A/B) of the particle-size peak top(A) to the particle-size peak top (B) is 1.0×10⁻² or more and 1.0×10 orless; and the fluorine ion concentration (mass %) is 500 ppm or lessbased on the solid-content mass of the fluorine-based polymerelectrolyte.

If such an electrolyte solution is used, an electrolyte solution havinga fluorine-based electrolyte highly dispersed therein and low fluorineion concentration can be provided. In addition, if such an electrolytesolution is used, an electrolyte membrane and electrode catalyst layerhaving high hot water dissolution resistance can be provided.

The method for producing an electrolyte solution of the secondembodiment, although it is not particularly limited, is, for example, amethod of dissolving a polymer electrolyte by supplying a solventcomprising particles of a polymer electrolyte and water to a closedreactor such as an autoclave made of SUS316, replacing the interioratmosphere of the autoclave with an inert gas such as nitrogen; andheating the internal solution while stirring. Alternatively, in otherembodiment, a production method of continuously dissolving a polymerelectrolyte by the aforementioned dissolution facility is mentioned. Inview of high chemical durability and productivity, the latter productionmethod is preferable.

Now, these electrolyte solutions will be more specifically describedbelow.

(Fluorine-Based Polymer Electrolyte)

If a fluorine-based polymer electrolyte having an —SO₃X group is used,the solubility of the polymer electrolyte to a solvent is more improved.Furthermore, as the fluorine-based polymer electrolyte, although it isnot particularly limited, for example, a fluorine-based polymerelectrolyte containing a copolymer having a repeating unit representedby the following formula (1) and a repeating unit represented by thefollowing formula (2), is preferable.—(CFZ—CF₂)—  (1)

(in the above formula (1), Z represents H, Cl, F or a perfluoroalkylgroup having 1 to 3 carbon atoms).—(CF₂—CF(—O—(CF₂CF(CF₃)O)_(n)—(CF₂)_(m)—SO₃X))—  (2)

(in the above formula (2), X is hydrogen, an alkali metal, analkaline-earth metal, or NR₁R₂R₃R₄ where R₁, R₂, R₃ and R₄ are eachindependently an alkyl group having 1 to 3 carbon atoms or hydrogen; mis an integer of 0 to 12; and n is an integer of 0 to 2, with theproviso that m and n are not simultaneously 0).

Among them, a fluorine-based polymer electrolyte in which Z is F, X isK, Na or Li, n is 0 and m is 2 is preferable. Furthermore, afluorine-based polymer electrolyte in which Z is F, X is Na, n is 0 andm is 2 is more preferable. If such a fluorine-based polymer electrolyteis used, dispersibility tends to be more improved.

The fluorine-based polymer electrolyte may have other functional groups.Examples of other functional groups include, but are not particularlylimited to, an —SO₂F group, a —CF₂H group and a —CF₃ group.

The average particle diameter of a fluorine-based polymer electrolyte inan electrolyte solution obtained by the dynamic light scatteringparticle-size measurement is preferably 10 nm or more and less than 500nm, more preferably 50 nm or more and 300 nm or less and further averageparticle diameter of a fluorine-based polymer electrolyte falls withinthe above range, the stability of particles of the fluorine-basedpolymer electrolyte is improved and particles of the fluorine-basedpolymer electrolyte can be easily produced. Note that the averageparticle diameter can be determined by the dynamic light scatteringparticle-size measurement described in Examples.

(Equivalent Mass)

The equivalent mass of a fluorine-based polymer electrolyte in anelectrolyte solution is preferably 300 to 1,000 g/eq, more preferably400 to 900 g/eq and further preferably 500 to 800 g/eq. If theequivalent mass is 300 g/eq or more, e.g., an electrolyte membranehaving further excellent power generation performance tends to beobtained. In contrast, if the equivalent mass is 1,000 g/eq or less,e.g., an electrolyte membrane having lower water-absorbing property andmore excellent mechanical strength tends to be obtained. The “equivalentmass of a fluorine-based polymer electrolyte” herein refers to a drymass per equivalent of a sulfonate group. Note that equivalent mass of afluorine-based polymer electrolyte can be measured by the methoddescribed in Examples (described later).

(Solid-Content Concentration)

The solid-content concentration of a fluorine-based polymer electrolytein an electrolyte solution is preferably 11 to 50 mass %, morepreferably 15 to 45 mass % and further preferably 20 to 40 mass %. Ifthe solid-content is 11 mass % or more, the yield per unit time tends tobecome more excellent. In contrast, if the solid-content is 50 mass % orless, difficulty of handling due to an increase in viscosity tends to bemore suppressed. The solid-content concentration can be measured by themethod described in Examples.

(Melt Flow Rate)

As an index for the degree of polymerization of a fluorine-based polymerelectrolyte in an electrolyte solution, a melt flow rate (hereinafterreferred to also as “MFR”) can be used. The MFR of a fluorine-basedpolymer electrolyte is preferably 100 g/10 minutes or less, morepreferably 10 g/10 minutes or less and further preferably 5 g/10 minutesor less. If MFR is 100 g/10 minutes or less, the output characteristicsof a fuel cell tend to be successfully maintained for a longer time.Furthermore, MFR is preferably 0.01 g/10 minutes or more, morepreferably 0.1 g/10 minutes or more and further preferably 0.3 g/10minutes or more. If the MFR is 0.01 g/10 minutes or more, afluorine-based polymer electrolyte tends to be successfully and moreeffectively dissolved. Note that MFR can be measured by the methoddescribed in Examples.

(Spherical Shape)

The particles of a fluorine-based polymer electrolyte in an electrolytesolution are preferably spherical. The “spherical” herein refers to ashape having an aspect ratio of 3.0 or less. Generally, as the aspectratio of a shape becomes closer to 1.0, the shape becomes closer to asphere. The aspect ratio of the spherical particles is preferably 3.0 orless, more preferably 2.0 and further preferably 1.5. The lower limit ofthe aspect ratio of the spherical particles is preferably 1.0. If theaspect ratio falls within the above range, the viscosity of theelectrolyte solution further decreases and handling tends to be improvedeven if the solid-content mass of a fluorine-based polymer electrolyteis increased. Note that the aspect ratio can be measured by the methoddescribed in Examples.

(Solvent)

Examples of the water-containing solvent include, but not particularlylimited to, water or a water-organic solvent mixed solvent. Examples ofsuch an organic solvent include, but not particularly limited to, proticorganic solvents such as an alcohol and glycerin; and aprotic solventssuch as N,N-dimethylformamide, N,N-dimethylacetamide andN-methylpyrrolidone. These can be used alone or in combination of two ormore.

As the alcohol, although it is not particularly limited, for example, alow-boiling point alcohol having 1 to 3 carbon atoms is preferable.These alcohols may be used alone or in combination of two or more.Examples of the low boiling-point alcohol having 1 to 3 carbon atomsinclude, but not particularly limited to, at least one alcohol selectedfrom the group consisting of methanol, ethanol, 1-propanol and2-propanol. Among them, ethanol and 1-propanol are preferable. If suchan alcohol is used, the dispersibility of a fluorine-based polymerelectrolyte in an electrolyte solution tends to be more improved and theaffinity of the fluorine-based polymer electrolyte tends to be moreimproved.

The content of water in a solvent is preferably 80 to 100 mass %, morepreferably 90 to 100 mass % or more and further preferably 100 mass %.If the concentration of water is the above concentration, thedispersibility of a fluorine-based polymer electrolyte in an electrolytesolution tends to be more improved. Because of this, the solid-contentmass of the fluorine-based polymer electrolyte tends to be successfullyimproved.

The content of an organic solvent in the solvent is preferably 0 to 50mass %, more preferably 0 to 20 mass %, further preferably 0 to 10 mass% and still further preferably 0 mass %. If the concentration of anorganic solvent is the above concentration, the dispersibility of afluorine-based polymer electrolyte in an electrolyte solution tends tobe more improved. Because of this, the solid-content mass of thefluorine-based polymer electrolyte tends to be successfully increased.Furthermore, using no flammable organic solvent is preferable in asafety point of view.

(Fluorine Ion Concentration)

The fluorine ion concentration (mass %) is used for indicating degree ofdecomposition of a fluorine-based polymer electrolyte in an electrolytesolution. More specifically, if the concentration of a fluorine ion inan electrolyte solution is high, the decomposition of a fluorine-basedpolymer electrolyte has proceeded. Because of this, the fluorine ionconcentration of an electrolyte solution is 500 ppm or less based on thesolid-content mass of a fluorine-based polymer electrolyte, preferably300 ppm or less and more preferably 200 ppm or less. Furthermore, thefluorine ion concentration is preferably lower, more preferably 0.10 ppmor more and further preferably 0 ppm or more. If the fluorine ionconcentration is 500 ppm or less, the decomposition amount afluorine-based polymer electrolyte is not large. Thus, the electrolytemembrane and electrode produced by the electrolyte solution have moreexcellent hot water dissolution resistance. The fluorine ionconcentration can be increased by increasing dissolution time ordissolution temperature and decreased by decreasing dissolution time ordissolution temperature. Note that fluorine ion concentration can bemeasured by the method described in Examples later described.

The thermal decomposition initiation temperature of a fluorine-basedpolymer electrolyte is preferably 150° C. or more, more preferably 250°C. or more and further preferably 350° C. or more. The upper limit ofthe thermal decomposition initiation temperature of a fluorine-basedpolymer electrolyte, although it is not particularly limited, ispreferably higher but the thermal decomposition initiation temperatureof polytetrafluoroethylene, i.e., 390° C. or less. If the thermaldecomposition initiation temperature falls within the above range, thefluorine ion concentration of an electrolyte solution tends to be lower.The thermal decomposition initiation temperature of a fluorine-basedpolymer electrolyte is increased if X of —SO₃X of the above formula (2)is replaced to obtain H-type; and decreased if X of —SO₃X of the aboveformula (2) is replaced to obtain a salt type. Note that such a thermaldecomposition initiation temperature can be measured by a differentialheat/thermogravimetric simultaneous measurement device.

[Scattering Intensity Ratio in Dynamic Light Scattering Particle-SizeMeasurement]

In the dynamic light scattering particle-size measurement, with respectto an electrolyte solution, at least one particle-size peak top (A)within the range of 0.1 or more to less than 5.0 μm and at least oneparticle-size peak top (B) within the range of 5.0 or more and 50.0 μmor less are obtained. The scattering intensity ratio (A/B) of theparticle-size peak top (A) to the particle-size peak top (B) is 1.0×10⁻²or more and 1.0×10 or less. The scattering intensity ratio in thedynamic light scattering particle-size measurement is used fordetermining dispersibility of a polymer electrolyte in an electrolytesolution, in other words, can be used as a measure of dissolution. Thescattering intensity ratio can be measured by the method described inExamples described later.

The ratio (A/B) of a scattering intensity (1/nm) is preferably 1.0×10⁻²or more and 1.0×10 or less, more preferably 1.0×10⁻¹ or more and 5.0 orless and further preferably 5.0×10⁻¹ or more and 2.0 or less. If thescattering intensity ratio is 1.0×10 or less, a polymer electrolyte issufficiently dissolved and dispersibility is further improved. In short,the larger the particle-size peak (B) than (A), the more thedispersibility tends to be improved. This was experimentally found bythe dynamic light scattering particle-size measurement of theelectrolyte solutions sampled with the passage of time; however thereason for this is not found. The scattering intensity ratio of 1.0×10⁻²or less means that a polymer electrolyte is conceivably decomposed andreduced in molecular mass; however it does not means only thisphenomenon. The scattering intensity ratio A/B can be increased bydecreasing the residence time or dissolution temperature and decreasedby increasing the residence time or dissolution temperature.

The difference in particle size between the particle-size peak top (A)and the particle-size peak top (B) is preferably 1 to 49.9 μm, morepreferably 5 to 40 μm and further preferably 10 to 30 μm. If thedifference in particle size between the particle-size peak top (A) andthe particle-size peak top (B) falls within the above range, a polymerelectrolyte is sufficiently dissolved and dispersibility tends to bemore increased.

[Transmittance in UV Measurement]

In the present embodiment, in addition to the scattering intensity ratioobtained in the dynamic light scattering particle-size measurement,transmittance in UV measurement of a solution having a solid-content of20 mass % at a wavelength of 800 nm also can be used as a base fordetermining dissolution. The transmittance of the electrolyte solutionis preferably 90% T or more, preferably 95% T or more and preferably 98%T or more. If the transmittance is 90% T or more, the polymerelectrolyte is sufficiently dissolved and dispersibility tends to behigh. The UV measurement can be performed by the method described inExamples described later.

[Scattering Peak in Laser Diffraction/Scattering Particle SizeDistribution Measurement]

It is preferable that, in the laser diffraction/scattering particle sizedistribution measurement, no scattering peak is observed with respect tothe electrolyte solution. Based on the presence or absence of ascattering peak in the laser diffraction/scattering particle sizedistribution measurement, whether dissolution sufficiently proceeds ornot can be determined. In other words, if a scattering peak is present,it is suggested that a polymer electrolyte is not sufficiently dissolvedor remains undissolved. In contrast, if a scattering peak is notpresent, it is suggested that a polymer electrolyte is sufficientlydissolved. In the laser diffraction/scattering particle sizedistribution measurement, the scattering peak can be measured by themethod described in Examples described later.

[Electrolyte Solution (Third Embodiment)]

The electrolyte solution of the third embodiment contains afluorine-based polymer electrolyte. Forty % or more of a polymerelectrolyte chain terminal of the fluorine-based polymer electrolyte is—CF₂H. The fluorine ion concentration (mass %) is 0.10 to 500 ppm basedon the solid-content mass of the fluorine-based polymer electrolyte. TheFe concentration is 0.010 to 10 ppm based on the solid-content mass ofthe fluorine-based polymer electrolyte.

If such an electrolyte solution is used, an electrolyte membrane andelectrode catalyst layer having high hot water dissolution resistancecan be provided.

Examples of the method for producing an electrolyte solution accordingto the third embodiment include, but not particularly limited to, amethod of dissolving a polymer electrolyte by placing particles of apolymer electrolyte and water-containing solvent in a closed reactorsuch as an autoclave made of SUS316, substituting the interioratmosphere of the autoclave with an inert gas such as nitrogen; andheating the internal solution while stirring. As another method, aproduction method by continuously dissolving a polymer electrolyte inthe aforementioned dissolution facility is mentioned. In view of highchemical durability and productivity, the latter production method ispreferable.

(Ratio of —CF₂H Group)

Examples of the structure of a polymer chain terminal of afluorine-based polymer electrolyte contained in an electrolyte solution,include a —CF₂H group, a —CF₃ group, a —COOH group and a —COONa group.Among them, a —CF₂H group is preferable. The amount of —CF₂H group basedon the total number of polymer chain terminals of a fluorine-basedpolymer electrolyte is preferably 40% or more, more preferably 50% ormore and further preferably 90% or more. If 40% or more of the polymerchain terminals consists of —CF₂H group, the Fenton tolerance is moreimproved compared to the electrolytes having a —COOH group or a —COONagroup at a terminal, and the chemical durability of the resultant fuelcell tends to be more improved. In contrast, if 40% or more of thepolymer chain terminals consists of a —CF₂H group, since an additionalproduction process after the fluorination step and the like is notrequired compared to electrolyte having a —CF₃ group at a terminal,productivity tends to be more improved.

(Fe Concentration)

The concentration (mass %) of Fe contained in an electrolyte solutionbased on the solid-content mass of a fluorine-based polymer electrolyteis 0.010 ppm or more and 10 ppm or less, preferably 0.050 ppm or moreand 5 ppm or less and more preferably 0.10 ppm or more and 1 ppm orless. If the Fe concentration is 10 ppm or less, the concentration ofFe, which induces generation of radicals during operation of a fuelcell, is low, with the result that deterioration of the electrolytemembrane is suppressed and the chemical durability of a fuel cell tendsto be more improved. In contrast, if the Fe concentration is 0.01 ppm ormore, the electrolyte solution and membrane can be produced without astep of removing Fe, and productivity tends to be more improved.

Note that electrolyte solutions having characteristics of both the firstembodiment and second embodiment, electrolyte solutions havingcharacteristics of both the first embodiment and third embodiment,electrolyte solutions having characteristics of both the secondembodiment and third embodiment, and electrolyte solutions havingcharacteristics of all of the first embodiment, second embodiment andthird embodiment are included in the range of the electrolyte solutionof the present embodiment.

[Electrolyte Membrane (First Embodiment)]

The electrolyte membrane of the first embodiment is formed of anelectrolyte solution as mentioned above. Examples of a method forproducing the electrolyte membrane of the first embodiment include, butnot particularly limited to, a method having a step of applying anelectrolyte solution obtained above to a substrate, a step of drying theelectrolyte solution applied to the substrate to obtain an electrolytemembrane and a step of removing the electrolyte membrane from thesubstrate. In this manner, the electrolyte membrane can be produced. Themethod of producing an electrolyte membrane as mentioned above is calledas a cast film-forming method. A film can be obtained by spreading anelectrolyte solution in a reactor, for example, a petri dish, heatingthe dish in e.g., an oven as needed to evaporate at least part of asolvent, and then peeling the film from the reactor. Alternatively, asheet-form coating film can be obtained by applying an electrolytesolution to e.g., a glass plate or a film such that the thickness iscontrolled to be uniform by a device such as a blade coater, a gravurecoater or a comma coater having a mechanism such as a blade, an airknife or a reverse roll in accordance with the manner of a cast filmformation. Furthermore, a film is continuously formed by continuouscasting to obtain a long sheet like film.

Examples of the film serving as a substrate include, but are notparticularly limited to, poly(ethylene terephthalate) (PET),poly(butylene terephthalate) (PBT), polyethylene naphthalate (PEN),polyester including a liquid crystal polyester, triacetyl cellulose(TAC), polyarylate, polyether, polycarbonate (PC), polysulfone,polyether sulfone, cellophane, aromatic polyamide, polyvinyl alcohol,polyethylene (PE), polypropylene (PP), poly(vinyl chloride) (PVC),polystyrene (PS), an acrylonitrile-butadiene-styrene copolymer (ABS),polymethyl methacrylate (PMMA), polyamide, polyacetal (POM),poly(phenylene ether) (PPE), poly(phenylene sulfide) (PPS),polyamideimide (PAI), polyether amide (PEI), polyetheretherketone(PEEK), polyimide (PI), polymethylpentene (PMP),polytetrafluoroethylene, (PTFE), fluorinated ethylene-propylene (FEP), atetrafluoroethylene-ethylene (ETFE) copolymers, poly(vinylidenefluoride) (PVDF), polybenzazole (PBZ), polybenzoxazole (PBO),polybenzothiazole (PBT), polybenzimidazole (PBI) and poly(paraphenyleneterephthalic imide) (PPTA).

[Electrolyte Membrane (Second Embodiment)]

The electrolyte membrane of the second embodiment contains afluorine-based polymer electrolyte. Forty % or more of a polymerelectrolyte chain terminal of the fluorine-based polymer electrolyte is—CF₂H. The fluorine ion concentration (mass %) is 0.10 to 500 ppm basedon the solid-content mass of the fluorine-based polymer electrolyte. TheFe concentration (mass %) is 0.01 to 10 ppm based on the solid-contentmass of the fluorine-based polymer electrolyte.

If the amount of fluorine-based polymer electrolyte chain terminal andthe amount of Fe are set at predetermined amounts, the chemicaldurability of the fuel cell is more improved.

A method for producing such an electrolyte membrane, although it is notparticularly limited, may be a method of dissolving a polymerelectrolyte by placing particles of a polymer electrolyte and a solventcontaining water to a closed reactor such as an autoclave made ofSUS316; substituting the interior atmosphere of the autoclave with aninert gas such as nitrogen; and heating the internal solution whilestirring. As another method, a production method by continuouslydissolving a polymer electrolyte in the aforementioned dissolutionfacility is mentioned. In view of high chemical durability andproductivity, the latter production method is preferable.

(Ratio of —CF₂H group)

Examples of the structure of a polymer chain terminal of afluorine-based polymer electrolyte contained in an electrolyte solution,include a —CF₂H group, a —CF₃ group, a —COOH group and a —COONa group.Among them, —CF₂H group is preferable. The amount of —CF₂H group basedon the total number of polymer chain terminals of a fluorine-basedpolymer electrolyte is preferably 40% or more, more preferably 50% ormore and further preferably 90% or more. If 40% or more of the polymerchain terminals consists of —CF₂H group, the Fenton tolerance is moreimproved compared to the electrolyte membrane having a —COOH group or a—COONa group at a terminal, and the chemical durability of the resultantfuel cell tends to be more improved. In contrast, if 40% or more of thepolymer chain terminals consists of a —CF₂H group, since an additionalproduction process after the fluorination step and the like is notrequired compared to an electrolyte membrane having a —CF₃ group at aterminal, productivity tends to be more improved.

(Fe Concentration)

The concentration (mass %) of Fe contained in an electrolyte membranebased on the solid-content mass of the fluorine-based polymerelectrolyte is 0.010 ppm or more and 10 ppm or less, preferably 0.050ppm or more and 5 ppm or less and more preferably 0.10 ppm or more and 1ppm or less. If the Fe concentration is 10 ppm or less, theconcentration of Fe, which induces generation of radicals duringoperation of a fuel cell, is low, with the result that deterioration ofthe electrolyte membrane is suppressed and the chemical durability of afuel cell tends to be more improved. In contrast, if the Feconcentration is 0.01 ppm or more, the electrolyte solution and membranecan be produced without a step of removing Fe, and productivity tends tobe more improved.

Note that electrolyte membranes having characteristics of both the firstembodiment and second embodiment, electrolyte membranes havingcharacteristics of both the first embodiment and third embodiment,electrolyte membranes having characteristics of both the secondembodiment and third embodiment, and electrolyte membranes havingcharacteristics of all of the first embodiment, second embodiment andthird embodiment are included in the range of the electrolyte membraneof the present embodiment.

[Electrode Catalyst Layer]

The electrode catalyst layer according to the present embodiment isformed of the electrolyte solution of the first embodiment or the secondembodiment. The electrode catalyst layer according to the presentembodiment can contain composite particles containing fine particles ofa catalytic metal and a conductive agent, and a fluorine-based polymerelectrolyte contained in the electrolyte solution serving as a binder.Furthermore, the electrode catalyst layer may contain a water repellentagent as needed.

Examples of the catalytic metal to be used in the electrode catalystlayer include, but not particularly limited to, a metal accelerating anoxidation reaction of hydrogen and a reductive reaction of oxygen.Preferable specific examples of such a metal include, but notparticularly limited to, at least one metal selected from the groupconsisting of platinum, gold, silver, palladium, iridium, rhodium,ruthenium, iron, cobalt, nickel, chromium, tungsten, manganese, vanadiumand alloys of these. Among them, principally, platinum is used.

As the conductive agent, although it is not particularly limited as longas it consists of particles having conductivity (conductive particles),for example, at least one type of conductive particle selected from thegroup consisting of carbon black such as furnace black, channel blackand acetylene black and activated carbon, graphite and various type ofmetals, is preferable.

The particle diameter of the conductive agents is not particularlylimited, preferably 10 angstroms to 10 μm, more preferably 50 angstromsto 1 μm and further preferably 100 to 5,000 angstroms. If the particlediameter falls within the above range, the surface area is increased andthe effect of efficiently dispersing and carrying catalytic metal fineparticles tends to be more improved.

The particle diameter of a catalytic metal fine particles (electrodecatalyst particles), although it is not particularly limited, ispreferably 10 to 1,000 angstroms, more preferably 10 to 500 angstroms,further preferably 15 to 100 angstroms. If the particle diameter fallswithin the above range, the surface area is increased and the contactarea with a binder electrolyte tends to be further and successfullyincreased.

The composite particles contain the catalytic metal particles in anamount of preferably 1 to 99 mass %, more preferably 10 to 90 mass % andfurther preferably 30 to 70 mass % based on the conductive particles.More specifically, Pt catalyst-carrying carbon such as TEC10E40Emanufactured by Tanaka Kikinzoku Kogyo k. k. can be mentioned as asuitable example. If the content falls within the above range, a desiredcatalyst activity tends to be more easily obtained.

The amount of catalyst carried by the electrode catalyst layer based onthe electrode area in the state where an electrode catalyst layer isformed, is preferably 0.001 to 10 mg/cm², more preferably 0.01 to 5mg/cm² and further preferably 0.1 to 1 mg/cm². The thickness of theelectrode catalyst layer is preferably 0.01 to 50 μm, more preferably0.1 to 30 μm and further preferably 1 to 20 μm. If the amount ofcatalyst and the thickness fall within the above ranges, an electrodecatalyst layer having such a sufficient amount of carrier that can showsufficient power generation performance, can be easily formed; at thesame time, a reduction in diffusivity of a gas within the electrodecatalyst layer tends to be successfully suppressed.

The porosity of the electrode catalyst layer, although it is notparticularly limited, is preferably 10 to 90 vol %, more preferably 20to 80 vol % and further preferably 30 to 60 vol %. If the porosity fallswithin the above range, protonic conductivity is more satisfactorilyobtained; at the same time, diffusivity of a fuel gas and watergenerated by the power generation tends to be easily improved.

To improve water repellency, the electrode catalyst layer may furthercontain polytetrafluoroethylene (hereinafter referred to as “PTFE”). Inthis case, the shape of PTFE is not particularly limited as long as itpreferable. These may be used alone or as a mixture. When the electrodecatalyst layer contains PTFE, the content of PTFE based on the totalmass of the electrode catalyst layer is preferably 0.001 to 20 mass %,more preferably 0.01 to 10 mass % and further preferably 0.1 to 5 mass%. If the content falls within the above range, the water repellencytends to become more excellent.

To improve hydrophilicity, the electrode catalyst layer of the presentembodiment may further contain a metal oxide. In this case, as the metaloxide, although it is not particularly limited to, for example, at leastone metal oxide selected from the group consisting of Al2O3, B2O3, MgO,SiO2, SnO2, TiO2, V2O5, WO3, Y2O3, ZrO2, Zr2O3 and ZrSiO4, ispreferable. Of them, at least one metal oxide selected from the groupconsisting of Al2O3, SiO2, TiO2 and ZrO2 is more preferable and SiO2 isfurther preferable.

In the present embodiment, in the case where an electrode catalyst layercontains a metal oxide, the content of the metal oxide based on thetotal mass of the electrode catalyst layer is preferably 0.001 to 20mass %, more preferably 0.01 to 10 mass % and further preferably 0.1 to5 mass %. As the shape of a metal oxide, particulate and fibrous formcan be used. Among them, particularly infinite from is preferable. The“infinite” herein means that metal oxides of particulate or fibrous formare not seen by use of an optical microscope and an electron microscope.In particular, it is meant that even if an image of the electrodecatalyst layer is magnified up to six-figure times by a scanningelectron microscope (SEM), particulate and fibrous form metal oxides arenot observed; and that even if an image of the electrode catalyst layeris magnified up to six-figure times to several millions times by atransmission electron microscope (TEM), particulate and fibrous formmetal oxides are not clearly observed. Likewise, the “infinite” meansthat particulate and fibrous form metal oxides are not observed withinthe range of current microscopic techniques.

If the aforementioned electrode catalyst layer is used, a fuel cellrarely causing flooding and providing high output can be obtained. Thisis probably because the water content is successfully reduced and thedrainage of the electrode becomes excellent.

The electrode catalyst composition for forming the electrode catalystlayer may be used, if necessary, by further adding a solvent. Examplesof the solvent that can be used, although it is not particularlylimited, include water; alcohols such as ethanol, 2-propanol, ethyleneglycol and glycerin; CFC, HCFC, and HFC; of mixtures of these solvents.The addition amount of such a solvent based on the total mass of theelectrode catalyst composition is preferably 0.1 to 90 mass %, morepreferably 1 to 50 mass % and further preferably 5 to 20 mass %.

Examples of a method for producing an electrode catalyst layer include,but not particularly limited to, a method having a step of preparing anelectrode catalyst composition by dispersing a composite particlecontaining a catalytic metal and a conductive agent in an electrolytesolution as mentioned above, a step of applying the electrode catalystcomposition to a substrate and a step of drying the electrode catalystcomposition applied to the substrate to obtain the electrode catalystlayer. More specifically, the electrode catalyst layer can be formed bypreparing an electrode catalyst composition by dispersing a compositeparticle in an electrolyte solution and applying the composition ontothe electrolyte membrane or a substrate such as a PTFE sheet, and thendrying the composition to solidify it. Note that, the electrode catalystcomposition in this embodiment can be applied in accordance with anymethod generally known in the art such as a screen printing method and aspray method.

Alternatively, the electrode catalyst layer of the present embodimentcan be also obtained by applying the electrode catalyst composition asmentioned above to a gas diffusion electrode such as ELAT (registeredtrade mark, manufactured by BASF), which is formed by laminating a gasdiffusion layer and an electrode catalyst layer, by coating or dipping,followed by drying to solidify the composition.

[Membrane Electrode Assembly]

The membrane electrode assembly according to this embodiment has anelectrolyte membrane and an electrode catalyst layer as mentioned above.The membrane electrode assembly in the present embodiment (hereinafterreferred to as “MEA”) refers to an assembly unit prepared by joining twotypes of electrode catalyst layers, i.e., anode and cathode, to twosurfaces of the electrolyte membrane, respectively.

Examples of a method for producing the membrane electrode assemblyinclude, but not particularly limited to, a method of producing amembrane electrode assembly by laminating the electrolyte membraneobtained by a production method as mentioned above and electrodecatalyst layers. Alternatively, a membrane electrode assembly can bealso obtained by directly applying an electrode catalyst composition toan electrolyte membrane by coating or dipping, and then drying tosolidify the composition. Furthermore, a membrane electrode assembly canbe obtained by hot press of an electrolyte membrane and electrodecatalyst layers. Note that an assembly obtained by joining a pair of gasdiffusion layers on the outer side of electrode catalyst layers so as toface each other is sometimes called as MEA.

[Fuel Cell]

The fuel cell according to this embodiment has a membrane electrodeassembly as mentioned above. The MEA obtained as mentioned above isfurther combined to structural components such as a bipolar plate and abacking plate that are used in general solid polymer electrolyte fuelcells, to form a solid polymer electrolyte fuel cell. Such a solidpolymer electrolyte fuel cell is not limited as long as it has the samestructure as those known in the art except that the aforementioned MEAis employed as MEA. The bipolar plate refers to a plate formed of e.g.,a composite material of graphite and a resin or a metal and having agroove for supplying a gas such as a fuel and an oxidant in the surface.The bipolar plate has a function as a flow channel for supplying a fuelor an oxidant near an electrode catalyst, in addition to a function forallowing electrons to migrate into an external load circuit. MEA isinserted between these bipolar plates and a plurality of resultantstructures are laminated to produce a solid polymer electrolyte fuelcell according to this embodiment.

In the foregoing, embodiments for carrying out the present inventionhave been described; however, the present invention is not limited tothe present embodiments. The present invention can be modified invarious ways without departing from the spirit of the invention.

EXAMPLES

Now, the present invention will be more specifically described by way ofExamples and Comparative Examples; however the present invention is notlimited to these Examples alone. Note that the evaluation methods andmeasurement methods used in Examples and Comparative Examples are asfollows.

(1) Method for Measuring Melt Flow Rate (MFR) of Fluorine-Based PolymerElectrolyte

The melt flow rate (MFR g/10 minutes) of a fluorine-based polymerelectrolyte was measured based on JIS K-7210 by using a device having anorifice of 2.09 mm in inner diameter and 8 mm in length at a temperatureof 270° C. and a load of 2.16 kg.

(2) Method of Determining the Average Diameter of Particles of PolymerElectrolyte

The average particle diameter was obtained based on the followingdynamic light scattering particle-size measurement of an electrolytesolution.

(3) Method of Measuring the Aspect Ratio of Particles of PolymerElectrolyte

An aggregate of a polymer electrolyte obtained by applying an emulsionto e.g., aluminum foil and removing a solvent was observed by a scanningelectron microscope and an image thereof was taken. On the image, 20 ormore particles were selected and measured for major axis and minor axis.The ratios of major axes to minor axes were averaged to obtain theaspect ratio.

(4) Method of Measuring Equivalent Mass of Polymer Electrolyte

The polymer electrolytes obtained in Examples and Comparative Examples,if they were not H-type, were changed into H-type by substitution. Themembrane of the H-type polymer electrolyte (0.02 to 0.10 g) was dippedin 50 mL of a saturated aqueous NaCl solution (0.26 g/mL) of 25° C.,allowed to stand still for 10 minutes while stirring and subjected toneutralization titration using 0.01 N aqueous sodium hydroxide (specialgrade chemical, manufactured by Wako Pure Chemical Industries Ltd.)solution with phenolphthalein (special grade chemical, manufactured byWako Pure Chemical Industries Ltd.) used as an indicator.

More specifically, after neutralization, the resultant Na-type membranewas rinsed with pure water, dried under vacuum and weighed. Assumingthat the equivalent amount of sodium hydroxide required forneutralization was represented by M (mmol) and the mass of the Na-typemembrane was represented by W (mg), the equivalent mass (g/eq) wasobtained in accordance with the following formula.Equivalent mass=(W/M)−22(5) Method of Measuring the Solid-Content Concentration in Emulsion andElectrolyte Solution

The mass of a dried weighing cup was precisely measured at roomtemperature and represented by W0. Then an emulsion or an electrolytesolution (1 g) was placed in the weighing cup measured, preciselymeasured and represented by W1. The weighing cup having the emulsion orthe electrolyte solution placed therein was dried by a dryer (typeLV-120) manufactured by ESPEC CORP at a temperature of 200° C. for onehour or more and cooled in a desiccator containing silica gel. Aftercooled to room temperature, the mass of the weighing cup was preciselymeasured and represented by W2. The above operation was repeated threetimes. The solid-content concentration of a polymer electrolyte wasobtained in accordance with the following equation as an average of thevalues obtained by three operations.Solid-content concentration=(W2−W0)/(W1−W0)×100(6) Method of Measuring Concentration of Water in Emulsion orElectrolyte Solution

The concentration of water in emulsion or electrolyte solution wasmeasured by Karl Fischer moisture meter 841 Titrand (manufactured byMetrohm) with aquamicron dehydrating solvent MS (manufactured by APICorporation) used as a dehydrating solvent and HYDRANAL composite 5K(manufactured by Sigma Aldrich Japan) as Karl Fischer's reagent.

(7) Dynamic Light Scattering Particle-Size Measurement Method ofElectrolyte Solution and Method of Calculating Scattering IntensityRatio

To determine whether a polymer electrolyte was dissolved or not, i.e.,dispersibility, the dynamic light scattering particle-size of anelectrolyte solution was measured. When water alone was used as thesolvent of the electrolyte solution, a solution composition containing asolid-content of a polymer electrolyte (2.5 mass %) and water (97.5 mass%) was prepared by concentration or dilution operation, as a measurementsample. The dynamic light scattering particle-size was measured by aparticle size measurement system, ELS-Z2 plus apparatus (manufactured byOtsuka Electronics Co., Ltd.). More specifically, a measurement samplewas set in a disposable cell and irradiated with a semiconductor laser(30 mW, 658 nm). The intensity of 160° scattering light was measured byunit of photons/sec. Measurement was repeated 200 times in total and anaverage diameter of particles in a measurement sample and particle-sizepeaks were obtained. From the scattering intensities of the resultantparticle-size peaks, a scattering intensity ratio was obtained.

FIG. 2 shows distributions of the particles of fluorine-based polymerelectrolytes in electrolyte solutions in Reference Examples 1 to 4,Example 5 and Comparative Examples 2 and 3.

(8) Method of Measuring Transmittance by UV

To determine whether a polymer electrolyte was dissolved or not,transmittance of UV (a wavelength of 800 nm) through an electrolytesolution (a solid-content: 20 mass %) was measured by use of V-550manufactured by JASCO.

(9) Method of Measuring Scattering Peak in Laser Diffraction/ScatteringParticle Size Distribution Measurement of Electrolyte Solution

To determine the presence of a polymer electrolyte remainingundissolved, laser diffraction/scattering particle size distribution ofan electrolyte solution was measured by using a laserdiffraction/scattering particle size distribution measuring device,LA-950, manufactured by HORIBA Ltd. When the electrolyte solutioncontained bubbles, defoaming treatment was performed under a reducedpressure of −0.08 MPa before measurement.

(10) Method of Determining Clogging

An emulsion was supplied by a pump into a heated tube. Operation wascontinuously made so as to obtain a desired residence time. The behaviorof the solution discharged from a back pressure regulating valve wasobserved and the presence or absence of clogging was evaluated based onthe following criteria.

◯: Clogging was absent: a case where the solution is discharged fromback pressure regulating valve at a constant speed

Δ: Clogging tended to be present: a case where the speed of the solutiondischarged from back pressure becomes gradually slow

X: Clogging was present: a case where the solution is not dischargedfrom back pressure regulating valve

(11) Evaluation Method of Dissolution

Degree of dissolution was evaluated based on the following evaluationcriteria.

◯: Dissolution of a polymer electrolyte was sufficient: a case where thescattering intensity ratio in the dynamic light scattering particle-sizemeasurement is 1.0×10⁻² or more; a case where transmittance (% T) at awavelength of 800 nm in the UV measurement is 90% T or more; and a casewhere a scattering peak is present in the laser diffraction/scatteringparticle size distribution measurement

X: Dissolution of a polymer electrolyte was insufficient: a case exceptfor the above cases

(12) Method of Measuring Fluorine Ion Concentration in ElectrolyteSolution and Electrolyte Membrane

The fluorine ion concentration was measured by using a fluorine ionmeter manufactured by THERMO ORION and a fluorine composite electrode.More specifically, a calibration curve was prepared by using threefluorine ion concentrations of 0.1, 1 and 10 ppm; and the fluorine ionconcentration in an electrolyte solution was measured with reference tothe resultant calibration curve. Note that, if necessary, a measurementsample was diluted with the same solvent as used in an electrolytesolution and at the same dilution ratio, so as to fall within the rangeof the fluorine ion concentration and subjected to the measurement. Inthe case of a dilution sample, the resultant measurement value wascalculated based on the dilution rate and divided by the solid-contentmass of the electrolyte solution to obtain the fluorine ionconcentration in the solid-content mass of the fluorine-based polymerelectrolyte. In the case of an electrolyte membrane, an electrolyte wasdipped in a saturated aqueous solution of sodium chloride and theconcentration of fluorine ion eluted in the solution was measured in thesame manner as above.

(13) Method of Measuring Concentration of Fe in Electrolyte Solution andElectrolyte Membrane

An electrolyte solution or an electrolyte membrane was carbonized in anelectric furnace to obtain a carbide, which was washed with apredetermined amount of nitric acid. Fe in the wash solution wasquantified by ICP-AES (inductively coupled plasma emission spectrometer)to obtain the concentration of Fe.

(14) Quantitative Determination of Terminal-CF₂H of Electrolyte

The terminal-CF₂H of an electrolyte was quantified by NMR measurement.An electrolyte solution or an electrolyte membrane andN,N′-dimethylacetamide were placed in an outer tube of an NMR tube andheated at 80° C. To the inner tube of a double tube structure,deuterated dimethyl sulfoxide was placed. In this manner, an NMRmeasurement sample was prepared. The resultant sample was subjected to¹⁹F-NMR measurement using ECS400 manufactured by Jeol Resonance at ameasurement temperature of 120° C. Provided that the chemical shift ofthe main chain CF₂-chain signal was set at −119 ppm, the integral valuesof —CF₂—CF₂H derived signals observed at −137 ppm and −127 ppm wereobtained. Provided that the integral value of the same electrolyte lotdissolved at 300° C. for one hour in a batch system was regarded as100%, the ratio of the —CF₂H terminal group of an electrolyte in each ofExamples, Reference Examples and Comparative Examples was calculated.

(15) Method of Measuring Concentration of Alcohol in ElectrolyteSolution

The concentration of an alcohol in the electrolyte solution obtained ineach of Examples and Comparative Examples was measured by gaschromatography equipment G4000 (manufactured by Shimadzu Corporation)and a capillary column InertCap WAX (inner diameter: 0.25 mm, length: 30m, thickness: 0.25 μm) manufactured by GL Sciences Inc. Morespecifically, the concentration of an alcohol was measured as follows. Acalibration curve of an alcohol was previously prepared by using1-butanol (special grade chemical, manufactured by Wako Pure ChemicalIndustries Ltd.) as an internal standard substance. A measurement samplewas prepared by adding an electrolyte solution (1 g), a 1 mass % aqueous1-butanol solution (1 g) and purified water (18 g). The temperature ofan injection port was set at 200° C., the temperature of a hydrogenflame ionization detector was set at 210° C., and the temperature of anoven was set at 60° C. Thereafter, the measurement sample (1 μL) wasinjected by a microsyringe. Immediately after the injection, thetemperature of the oven was increased at a rate of 10° C./min. At thetime, a spectrum was measured. From the spectrum, the area of a peak wasobtained to measure the concentration of the alcohol.

(16) Method of Evaluating Hot Water Dissolution Resistance ofElectrolyte Membrane

An electrolyte membrane was allowed to stand still in a constanttemperature and humidity room (23° C. and 50% RH) for 24 hours.Thereafter, the mass of the membrane before treatment was measured.Subsequently, the electrolyte membrane was dipped in hot water of 90° C.and treated with heat for 5 hours. Subsequently, while the electrolytemembrane was dipped, the hot water was cooled. Thereafter, theelectrolyte membrane was taken out from the water, allowed to standstill in a constant temperature and humidity room (23° C. and 50% RH)for 24 hours and then, the mass of the electrolyte membrane was measuredto obtain the mass after treatment. The mass after treatment correspondsto the mass of the electrolyte after treatment. A mass loss ratio of theelectrolyte membrane was calculated in accordance with the followingequation. It is shown that the lower the mass loss ratio, the higher thehot water dissolution resistance.Mass loss ratio (%)=(mass before treatment−mass after treatment)/massbefore treatment×100(17) Evaluation of Fuel Cell

To examine battery characteristics (hereinafter referred to as “initialcharacteristics”) of the membrane electrode assembly prepared asdescribed later, a fuel cell was evaluated as follows.

First, an anode-side gas diffusion layer and a cathode-side gasdiffusion layer were allowed to face each other. Between these layers,MEA produced as described below was inserted and the resultant constructwas integrated into a cell for evaluation. As the anode-side andcathode-side gas diffusion layers, carbon cloth (ELAT (registered trademark) B-1, manufactured by DE NORA NORTH AMERICA of the United States)was used. The cell for evaluation was placed in an evaluation apparatus(manufactured by CHINO corporation) and increased in temperature to 80°C. Thereafter, hydrogen gas was supplied at a rate of 300 cc/min to theanode side; whereas air gas was supplied at a rate of 800 cc/min to thecathode side. Both the anode side and cathode side were pressurized to anormal pressure or 0.15 MPa (absolute pressure). These gases werepreviously humidified. Hydrogen gas and air gas were both humidified bya water bubbling method at a desired temperature and supplied to thecell for evaluation. Then, the cell for evaluation was maintained at acell temperature of 80° C. and at a voltage of 0.7 V for 20 hours undercondition under desired humidity conditions and thereafter the currentwas measured.

Example 1

Through the polymerization step, hydrolysis step and ultrafiltrationstep described in Example 1 of WO2011/034179, Na-type emulsion(solid-content: 35.0 mass %, water: 65.0 mass %) containing afluorine-based polymer electrolyte (equivalent mass=710 g/eq) consistingof a copolymer (MFR=3.2 g/10 minutes) of olefin fluoride (CF₂═CF₂) and avinyl fluoride compound (CF₂═CF—O—(CF₂)₂—SO₃Na) and having an averageparticle diameter of 111 nm and an aspect ratio of 1.0, was obtained.

The Na-type emulsion was supplied by a supply pump into a tube (thesurfer roughness of the inner wall=1 μm) made of Hastelloy C276 (Ni: 57mass %, Mo: 17 mass %, Cr: 16 mass %, Fe: 4-7 mass %, W: 3-4.5 mass %,Co≤2.5 mass %) and having an inner diameter of 2.17 mm, allowed to passthrough the tube placed in a thermostatic bath set at 290° C. anddischarge from a back pressure regulating valve set at 9 MPa to obtain ahomogeneous, colorless and transparent electrolyte solution AS1. Theresidential time of the emulsion in the tube placed in the thermostaticbath set at 290° C. was 7.5 minutes. The scattering intensity ratio A/Bof electrolyte solution AS1 in the dynamic light scatteringparticle-size measurement was 1.8. After electrolyte solution AS1 wasdiluted with water so as to have a solid-content of 20 mass %, thetransmittance of the solution at a wavelength of 800 nm (the UVmeasurement) was measured. The transmittance was 99.1% T. No laserscattering peak was observed with respect to electrolyte solution AS1.Terminal-CF₂H amount, fluorine ion concentration and Fe concentrationare as shown in Table 1.

Example 2

Continuous dissolution was carried out in the same manner as in Example1 except that the inner diameter of the tube described in Example 1 wasset at 7.53 mm and the residence time in the tube (the surface roughnessof the inner wall=5 μm) placed in the thermostatic bath was set at 15minutes. Homogeneous, colorless and transparent electrolyte solution AS2was obtained from the back pressure regulating valve. The scatteringintensity ratio A/B of electrolyte solution AS2 in the dynamic lightscattering particle-size measurement was 0.9. After electrolyte solutionAS2 was diluted with water so as to have a solid-content of 20 mass %,the transmittance of the solution at a wavelength of 800 nm (the UVmeasurement) was measured. The transmittance was 99.5% T. No laserscattering peak was observed with respect to electrolyte solution AS2.Terminal-CF₂H amount, fluorine ion concentration and Fe concentrationare as shown in Table 1.

Example 3

Continuous dissolution was carried out in the same manner as in Example1 except that the inner diameter of the tube described in Example 1 wasset at 44.8 mm and the residence time in the tube (the surface roughnessof the inner wall=5 μm) placed in the thermostatic bath was set at 45minutes. Homogeneous, colorless and transparent electrolyte solution AS3was obtained from the back pressure regulating valve. The scatteringintensity ratio A/B of electrolyte solution AS3 in the dynamic lightscattering particle-size measurement was 0.6. After electrolyte solutionAS3 was diluted with water so as to have a solid-content of 20 mass %,the transmittance of the solution at a wavelength of 800 nm (the UVmeasurement) was measured. The transmittance was 99.1% T. No laserscattering peak was observed with respect to electrolyte solution AS3.Terminal-CF₂H amount, fluorine ion concentration and Fe concentrationare as shown in Table 1.

Example 4

Continuous dissolution was carried out in the same manner as in Example1 except that the back pressure regulating valve described in Example 1was set at 6 MPa. Homogeneous, colorless and transparent electrolytesolution AS4 was obtained from the back pressure regulating valve. Thescattering intensity ratio A/B of electrolyte solution AS4 in thedynamic light scattering particle-size measurement was 2.0. Afterelectrolyte solution AS4 was diluted with water so as to have asolid-content of 20 mass %, the transmittance of the solution at awavelength of 800 nm (the UV measurement) was measured. Thetransmittance was 90.5% T. No laser scattering peak was observed withrespect to electrolyte solution AS4. The speed of the solution flowingout from the back pressure regulating valve became gradually slow;however, the speed did not reach 0 during the operation of 30 minutes.Furthermore, a safety valve set at 12 MPa was not actuated. From theabove, although tendency of clogging was slightly observed, it was ableto determine that dissolution can be made. Terminal-CF₂H amount,fluorine ion concentration and Fe concentration are as shown in Table 1.

Comparative Example 1

The Na-type emulsion described in Example 1 was subjected anH-conversion step and an ultrafiltration step to obtain an H-typeemulsion (solid-content: 30.1 mass %, water: 69.9 mass %) containing afluorine-based polymer electrolyte (equivalent mass=710 g/eq), whichconsisted of a copolymer (MFR=3.2 g/10 minutes) of olefin fluoride(CF₂═CF₂) and a vinyl fluoride compound (CF₂═CF—O—(CF₂)₂—SO₃H) and hadan average particle diameter of 111 nm and an aspect ratio of 1.0.Terminal-CF₂H amount, fluorine ion concentration and Fe concentrationare as shown in Table 1. Terminal-CF₂H amount and fluorine ionconcentration were small and the amount of Fe was large.

An autoclave made of Hastelloy C276 and having 300-mL in volume wascharged with the H-type emulsion (210 g). To the autoclave, nitrogen (2MPa) was introduced and the emulsion was dissolved at 165° C. for 5minutes while stirring at 600 rpm. The internal pressure of theautoclave increased as the temperature increased. The maximum pressurewas 2.7 MPa. After cooling, electrolyte solution AS5 taken out from theautoclave was cloudy. The scattering intensity ratio A/B of electrolytesolution AS5 in the dynamic light scattering particle-size measurementwas 1000. After electrolyte solution AS5 was diluted with water so as tohave a solid-content of 20 mass %, the transmittance of the solution ata wavelength of 800 nm (the UV measurement) was measured. Thetransmittance was 71% T. A laser scattering peak was observed withrespect to electrolyte solution AS5. From the above, it is consideredthat the emulsion remained in AS5 and the polymer electrolyte was notdissolved.

The above results are summarized in the following Table 1.

TABLE 1 Comparative Example 1 Example 2 Example 3 Example 4 Example 1Dissolution system Continuous Continuous Continuous Continuous Batchsystem system system system system Pressure (Mpa) 9 9 9 6 2.7Dissolution temperature (° C.) 290 290 290 290 165 Retention time(minutes) 7.5 15 45 7.5 7.5 A 1.8 0.9 0.6 2 1000 B 99.1 99.5 99.1 90.571 C Absent Absent Absent Absent Present Clogging ◯ ◯ ◯ Δ ◯ Dissolved ornot ◯ ◯ ◯ ◯ X Terminal CF2H amount 51 57 65 45 30 (%; based on totalnumber of terminals) Fluorine ion concentration 81 85 88 80 23 (ppm;based on the mass of fluorine-based polymer electrolyte) Feconcentration (ppm; based 0.21 0.40 0.62 0.18 10.5 on the mass offluorine-based polymer electrolyte) * A: Scattering intensity ratio (—)in dynamic light scattering particle-size measurement B: Transmittance(% T) at wavelength of 800 nm in UV measurement C: Presence or absenceof scattering peak in laser diffraction/scattering particle sizedistribution measurement

Reference Example 1

Through the polymerization step, hydrolysis step and ultrafiltrationstep described in Example 1 of WO2011-034179A1, an emulsion(solid-content: 32.0 mass %, water: 68.0 mass %) containing afluorine-based polymer electrolyte (equivalent mass=691 g/eq), whichconsisted of a copolymer (MFR=3.4 g/10 minutes) of olefin fluoride(CF₂═CF₂) and a vinyl fluoride compound (CF₂═CF—O—(CF₂)₂—SO₃K) and hadan average particle diameter of 139 nm and an aspect ratio of 1.0, wasobtained.

Subsequently, an autoclave made of SUS316 and having 300-mL in volumewas charged with the resultant emulsion (131.3 g) and distilled water(78.8 g)(manufactured by Wako Pure Chemical Industries Ltd.) to preparea raw material solution containing a fluorine-based polymer electrolyte(solid-content of 25 mass %) and water (75 mass %). To the autoclave,nitrogen was supplied so as to obtain 1.5 MPa (hereinafter, MPa refersto “gauge pressure”). The raw material solution was subjected to adissolution operation at 290° C. for 60 minutes while stirring at 600rpm. The internal pressure of the autoclave increased as the temperatureincreased. The maximum pressure was 6.9 MPa. After cooling, electrolytesolution AS6 taken out from the autoclave was homogeneous, colorless andtransparent. The scattering intensity ratio A/B of electrolyte solutionAS6 in the dynamic light scattering particle-size measurement was 1.2.The fluorine ion concentration of electrolyte solution AS6 was 123 ppm.No laser scattering peak was observed with respect to AS6. Theterminal-CF₂H amount and Fe concentration are as shown in Table 2.

Electrolyte solution AS6 was passed through a column packed with acation exchange resin to exchange K ions of the fluorine-based polymerelectrolyte with H ions to obtain electrolyte solution AS7. Electrolytesolution AS7 was poured onto a glass plate and applied (cast), placed inan oven and dried at 80° C. for 30 minutes and subsequently dried at120° C. for 30 minutes to remove the solvent. A heat treatment wasfurther applied at 160° C. for 20 minutes to obtain electrolyte membraneAM1 having a thickness of about 50 μm. When electrolyte membrane AM1 wassubjected to a hot water dissolution resistance test, the mass lossratio was 0.1 mass %.

An electrode catalyst layer was produced using electrolyte solution AS7as follows. To a Pt carrying carbon particle (0.70 g) (trade name“TEC10E40E” carrying Pt 36.0 mass %, manufactured by Tanaka KikinzokuKogyo k.k.), which was a composite particle consisting of conductivecarbon particles carrying a platinum (Pt) particle serving as a catalystparticle, AS7 (2.22 g) and ethanol (8.08 g) were added and these weresufficiently mixed by a homogenizer to obtain an electrode catalystcomposition. The electrode catalyst composition was applied onto a PTFEsheet in accordance with a screen printing method. After coating, theelectrode catalyst composition was dried in the air, at room temperaturefor one hour, and subsequently, dried at 160° C. for one hour. Asdescribed above, an electrode catalyst layer having a thickness of about10 μm was produced on the PTFE sheet. Of the electrode catalyst layersobtained in this manner, an electrode catalyst layer carrying Pt in anamount of 0.15 mg/cm² was used as an anode catalyst layer (thickness: 5μm) and an electrode catalyst layer carrying Pt in an amount of 0.30mg/cm² was used as a cathode catalyst layer (thickness: 10 μm).

The anode catalyst layer was placed so as to face the cathode catalystlayer. Electrolyte membrane AM1 was inserted between them and subjectedto hot press in the conditions of 180° C. and a surface pressure of 0.1MPa. In this manner, the anode catalyst layer and the cathode catalystlayer were transferred to the electrolyte membrane and joined to produceMEA.

MEA was subjected to fuel cell evaluation as previously described. As aresult, the current density after MEA was maintained at a celltemperature of 80° C. and a saturated vapor pressure at 80° C.(corresponding to a humidity of 100% RH) and a voltage of 0.7 V for 20hours, was 0.44 A/cm².

Reference Example 2

Through the polymerization step, hydrolysis step and ultrafiltrationstep described in Example 1 of WO2011-034179A1, an Na-type emulsion(solid-content: 36.7 mass %, water: 63.3 mass %) containing afluorine-based polymer electrolyte (equivalent mass=710 g/eq), whichconsisted of a copolymer (MFR=3.2 g/10 minutes) of olefin fluoride(CF₂═CF₂) and a vinyl fluoride compound (CF₂═CF—O—(CF₂)₂—SO₃Na) and hadan average particle diameter of 111 nm and an aspect ratio of 1.0, wasobtained.

Subsequently, an autoclave made of SUS316 and having 300-mL in volumewas charged with the Na-type emulsion (114.4 g) and distilled water(95.6 g)(manufactured by Wako Pure Chemical Industries Ltd.)(fluorine-based polymer electrolyte having a solid-content of 20 mass %and water of 80 mass %). To the autoclave, nitrogen was supplied so asto obtain 1.5 MPa. A dissolution operation was performed at 250° C. for120 minutes while stirring at 600 rpm. The internal pressure of theautoclave increased as the temperature increased. The maximum pressurewas 3.6 MPa. After cooling, electrolyte solution AS8 taken out from theautoclave was homogeneous, colorless and transparent. The scatteringintensity ratio A/B of electrolyte solution AS8 in the dynamic lightscattering particle-size measurement was 1.2. The fluorine ionconcentration of electrolyte solution AS8 was 101 ppm. No laserscattering peak was observed with respect to electrolyte solution AS8.The terminal-CF₂H amount and Fe concentration are as shown in Table 2.

Electrolyte solution AS8 was passed through a column packed with acation exchange resin to exchange Na ions of the fluorine-based polymerelectrolyte with H ions to obtain electrolyte solution AS9. The sameoperation as in Reference Example 1 was repeated using electrolytesolution AS9 to obtain electrolyte membrane AM2 having a thickness ofabout 51 μm. When electrolyte membrane AM2 was subjected to a hot waterdissolution resistance test, the mass loss ratio was 0.0 mass %.

The same operation as in Reference Example 1 was repeated usingelectrolyte solution AS9 to produce an anode catalyst layer and acathode catalyst layer. MEA was produced by using the anode catalystlayer, the cathode catalyst layer and electrolyte membrane AM2.

MEA was subjected to fuel cell evaluation as previously described. As aresult, current density after MEA was maintained at a cell temperatureof 80° C. and a saturated vapor pressure at 80° C. (corresponding to ahumidity of 100% RH) and a voltage of 0.7 V for 20 hours, was 0.45A/cm².

Reference Example 3

An autoclave made of SUS316 and having 300-mL in volume was charged withthe same Na-type emulsion (114.4 g) as in Reference Example 2, distilledwater (81.6 g) (manufactured by Wako Pure Chemical Industries Ltd.) andethanol of 14.0 g (fluorine-based polymer electrolyte having asolid-content of 20 mass %, water of 73.3 mass % and ethanol 6.7 mass%). To the autoclave, nitrogen was supplied so as to obtain 4.0 MPa. Adissolution operation was performed at 220° C. for 240 minutes whilestirring at 600 rpm. The internal pressure of the autoclave increased asthe temperature increased. The maximum pressure was 7.9 MPa. Aftercooling, electrolyte solution AS9 taken out from the autoclave washomogeneous, colorless and transparent. The scattering intensity ratioA/B of electrolyte solution AS9 in the dynamic light scatteringparticle-size measurement was 1.1. The fluorine ion concentration ofelectrolyte solution AS9 was 89 ppm. No laser scattering peak wasobserved with respect to electrolyte solution AS9. The terminal-CF₂Hamount and Fe concentration are as shown in Table 2.

Electrolyte solution AS9 was passed through a column packed with acation exchange resin to exchange Na ions of the fluorine-based polymerelectrolyte with H ions to obtain electrolyte solution AS10. The sameoperation as in Reference Example 1 was repeated using electrolytesolution AS10 to obtain electrolyte membrane AM3 having a thickness ofabout 51 μm. When electrolyte membrane AM3 was subjected to a hot waterdissolution resistance test, the mass loss ratio was 0.1 mass %.

The same operation as in Reference Example 1 was repeated usingelectrolyte solution AS10 to produce an anode catalyst layer and acathode catalyst layer. MEA was produced by using the anode catalystlayer, the cathode catalyst layer and electrolyte membrane AM3.

MEA was subjected to fuel cell evaluation as previously described. As aresult, current density after MEA was maintained at a cell temperatureof 80° C. and a saturated vapor pressure at 80° C. (corresponding to ahumidity of 100% RH) and a voltage of 0.7 V for 20 hours, was 0.45A/cm².

Reference Example 4

An autoclave made of SUS316 and having 300-mL in volume was charged withthe same Na-type emulsion (171.7 g) as in Reference Example 1 anddistilled water (38.3 g) (manufactured by Wako Pure Chemical IndustriesLtd.) (fluorine-based polymer electrolyte having a solid-content of 30mass % and water of 70 mass %). To the autoclave, nitrogen was suppliedso as to obtain 1.5 MPa. A dissolution operation was performed at 270°C. for 60 minutes while stirring at 600 rpm. The internal pressure ofthe autoclave increased as the temperature increased. The maximumpressure was 5.1 MPa. After cooling, electrolyte solution AS11 taken outfrom the autoclave was homogeneous, colorless and transparent. Thescattering intensity ratio A/B of electrolyte solution AS11 in thedynamic light scattering particle-size measurement was 7.0×10⁻¹. Thefluorine ion concentration of electrolyte solution AS11 was 100 ppm. Nolaser scattering peak was observed with respect to electrolyte solutionAS11. The terminal-CF₂H amount and Fe concentration are as shown inTable 2.

Electrolyte solution AS11 was passed through a column packed with acation exchange resin to exchange Na ions of the fluorine-based polymerelectrolyte with H ions to obtain electrolyte solution AS12. The sameoperation as in Reference Example 1 was repeated using electrolytesolution AS12 to obtain electrolyte membrane AM4 having a thickness ofabout 51 When electrolyte membrane AM4 was subjected to a hot waterdissolution resistance test, the mass loss ratio was 0.1 mass %.

The same operation as in Reference Example 1 was repeated usingelectrolyte solution AS12 to produce an anode catalyst layer and acathode catalyst layer. MEA was produced by using the anode catalystlayer, the cathode catalyst layer and electrolyte membrane AM4.

MEA was subjected to fuel cell evaluation as previously described. As aresult, current density after MEA was stored in the conditions of celltemperature: 80° C., saturated vapor pressure at 80° C. (correspondingto a humidity of 100% RH), and a voltage of 0.7 V, for 20 hours was 0.45A/cm².

Example 5

The same Na-type emulsion as in Reference Example 2 having asolid-content of 36.7 mass % was diluted with distilled watermanufactured by Wako Pure Chemical Industries Ltd. so as to obtain asolid-content in a polymer electrolyte of 35 mass % and stirred by asmall stirrer to be homogeneous. The diluted emulsion was supplied by asupply pump to a tube made of SUS316 and having an inner diameter of2.17 mm at a rate of 2.46 ml/min, allowed to pass through a tube havinga length of 10 m passing through a thermostatic bath set at 270° C. anddischarge from a back pressure regulating valve set at 7 MPa to obtain ahomogeneous, colorless and transparent electrolyte solution AS13. Thetime of supplying the emulsion into a thermostatic bath set at 270° was7.5 minutes. The scattering intensity ratio A/B of electrolyte solutionAS13 in the dynamic light scattering particle-size measurement was 1.8.The fluorine ion concentration of electrolyte solution AS13 was 79 ppm.No laser scattering peak was observed with respect to electrolytesolution AS13. The terminal-CF₂H amount and Fe concentration are asshown in Table 2.

Electrolyte solution AS13 was passed through a column packed with acation exchange resin to exchange Na ions of the fluorine-based polymerelectrolyte with H ions to obtain electrolyte solution AS14. The sameoperation as in Reference Example 1 was repeated using electrolytesolution AS14 to obtain electrolyte membrane AM5 having a thickness ofabout 50 μm. When electrolyte membrane AM5 was subjected to a hot waterdissolution resistance test, the mass loss ratio was 0.0 mass %.

The same operation as in Reference Example 1 was repeated usingelectrolyte solution AS14 to produce an anode catalyst layer and acathode catalyst layer. MEA was produced by using the anode catalystlayer, the cathode catalyst layer and electrolyte membrane AM5.

MEA was subjected to fuel cell evaluation as previously described. As aresult, current density after MEA was maintained at a cell temperatureof 80° C. and a saturated vapor pressure at 80° C. (corresponding to ahumidity of 100% RH) and a voltage of 0.7 V for 20 hours, was 0.46A/cm².

Comparative Example 2

As described in Japanese Patent Laid-Open No. 2010-225585, ComparativeExample 1, a flake-like electrolyte (water: 12.1 mass %) of afluorine-based polymer (equivalent mass=720 g/eq), which consisted of acopolymer (MFR=3.0 g/10 minutes) of olefin fluoride (CF₂═CF₂) and avinyl fluoride compound (CF₂═CF—O—(CF₂)₂—SO₃H). The flake-likeelectrolyte had various shapes. It was difficult to precisely measurethe sizes and aspect ratios under observation of an electron microscope;however the widths thereof generally fell within the range of 1 mm ormore.

An autoclave made of Hastelloy C and having 300-mL in volume was chargedwith the flake-like electrolyte (71.7 g) and distilled water (138.3g)(manufactured by Wako Pure Chemical Industries Ltd.) (fluorine-basedpolymer electrolyte having a solid-content of 30 mass % and water of 70mass %). To the autoclave, nitrogen was supplied so as to obtain 1.5MPa. A dissolution operation was performed at 240° C. for 120 minuteswhile stirring at 600 rpm. The internal pressure of the autoclaveincreased as the temperature increased. The maximum pressure was 3.2MPa. After cooling, electrolyte solution AS15 taken out from theautoclave was homogeneous, light brown and transparent. The scatteringintensity ratio A/B of electrolyte solution AS15 in the dynamic lightscattering particle-size measurement was 1.0. The fluorine ionconcentration of electrolyte solution AS15 was 706 ppm. No laserscattering peak was observed with respect to electrolyte solution AS15.The terminal-CF₂H amount and Fe concentration were as shown in Table 2.The fluorine ion content and Fe amount were large.

Electrolyte solution AS15 was passed through a column packed with a Naion exchange resin to exchange H ions of the fluorine-based polymerelectrolyte with Na ions to obtain electrolyte solution AS16. Thescattering intensity ratio A/B of electrolyte solution AS16 in thedynamic light scattering particle-size measurement was 2.1. The fluorineion concentration of electrolyte solution AS16 was 705 ppm. No laserscattering peak was observed with respect to electrolyte solution AS16.The same operation as in Reference Example 1 was repeated usingelectrolyte solution AS16 to obtain electrolyte membrane AM6 having athickness of about 52 μm. When electrolyte membrane AM6 was subjected toa hot water dissolution resistance test, the mass loss ratio was 3.4mass %.

The same operation as in Reference Example 1 was repeated usingelectrolyte solution AS16 to produce an anode catalyst layer and acathode catalyst layer. MEA was produced by using the anode catalystlayer, the cathode catalyst layer and electrolyte membrane AM6.

MEA was subjected to fuel cell evaluation as previously described. As aresult, current density after MEA was maintained at a cell temperatureof 80° C. and a saturated vapor pressure at 80° C. (corresponding to ahumidity of 100% RH) and a voltage of 0.7 V for 20 hours, was 0.04A/cm².

In Comparative Example 2, when the flake-like electrolyte was dissolvedat a high temperature, thermal decomposition took place and fluorine ionconcentration increased. This phenomenon showed that the electrolytemembrane and electrode catalyst layer prepared from the electrolytesolution of Comparative Example 2 each are low in hot water dissolutionresistance and significantly low in battery characteristic.

Comparative Example 3

The same operation as described in Comparative Example 1 of JapanesePatent Laid-Open No. 2010-225585 was repeated except that X of —SO₃X ofthe fluorine-based polymer electrolyte was Na to obtain a flake-likeelectrolyte (water: 8.8 mass %) of a fluorine-based polymer (equivalentmass=720 g/eq), which consisted of a copolymer (MFR=3.0 g/10 minutes) ofolefin fluoride (CF₂═CF₂) and a vinyl fluoride compound(CF₂═CF—O—(CF₂)₂—SO₃Na). The flake-like electrolyte had various shapes.It was difficult to precisely measure the sizes and aspect ratios underobservation of an electron microscope; however the widths thereofgenerally fall within the range of 1 mm or more.

An autoclave made of SUS316 and having 300-mL in volume was charged withthe flake-like electrolyte (46.1 g) and distilled water (164.0g)(manufactured by Wako Pure Chemical Industries Ltd.) (fluorine-basedpolymer electrolyte having a solid-content of 20 mass % and water of 80mass %). To the autoclave, nitrogen was supplied so as to obtain 1.5MPa. A dissolution operation was performed at 290° C. for 240 minuteswhile stirring at 600 rpm. The internal pressure of the autoclaveincreased as the temperature increased. The maximum pressure was 6.9MPa. After cooling, electrolyte solution AS17 taken out from theautoclave contained polymer electrolyte remaining unsolved and whiteturbidity. The scattering intensity ratio A/B of electrolyte solutionAS17 in the dynamic light scattering particle-size measurement was1.0×10². More specifically, electrolyte solution AS17 had peak A aloneand no Peak B. The fluorine ion concentration of electrolyte solutionAS17 was 145 ppm. Electrolyte solution AS17 had a laser scattering peak.The terminal-CF₂H amount and Fe concentration are as shown in Table 2.The Fe amount was large.

Electrolyte solution AS17 was filtered by use of a membrane filter of 10μm in diameter. The filtrate was subjected to the same operation as inReference Example 1 to obtain electrolyte membrane AM7 having athickness of about 51 μm. When electrolyte membrane AM7 was subjected toa hot water dissolution resistance test, the mass loss ratio was 1.9mass %.

The same operation as in Reference Example 1 was repeated usingelectrolyte solution AS17 filtered to produce an anode catalyst layerand a cathode catalyst layer. MEA was produced by using the anodecatalyst layer, the cathode catalyst layer and electrolyte membrane AM7.

MEA was subjected to fuel cell evaluation as previously described. As aresult, current density after MEA was maintained at a cell temperatureof 80° C. and a saturated vapor pressure at 80° C. (corresponding to ahumidity of 100% RH) and a voltage of 0.7 V for 20 hours, was 0.09A/cm².

In Comparative Example 3, even if a salt electrolyte bulk was dissolvedat a high temperature, a polymer electrolyte remained unsolved anddispersibility was low. It was also shown that the electrolyte membraneand electrode catalyst layer prepared from the electrolyte solution arelow in hot water dissolution resistance and significantly low in batterycharacteristics.

Comparative Example 4

An autoclave made of SUS316 and having 300-mL in volume was charged withthe same Na-type emulsion (114.4 g) as in Reference Example 2 anddistilled water (95.6 g)(manufactured by Wako Pure Chemical IndustriesLtd.) (fluorine-based polymer electrolyte having a solid-content of 20mass % and water of 80 mass %). To the autoclave, nitrogen was suppliedso as to obtain 1.5 MPa. A dissolution operation was performed at 360°C. for 60 minutes while stirring at 600 rpm. The internal pressure ofthe autoclave increased as the temperature increased. The maximumpressure was 15.0 MPa. After cooling, electrolyte solution AS18 takenout from the autoclave was slightly black and transparent. Thescattering intensity ratio A/B of electrolyte solution AS18 in thedynamic light scattering particle-size measurement was 1.0×10⁻³. Thefluorine ion concentration of electrolyte solution AS18 was 1530 ppm.Electrolyte solution AS18 had a laser scattering peak. The terminal-CF₂Hamount and Fe concentration are as shown in Table 2. The fluorine ioncontent and Fe amount were large.

Electrolyte solution AS18 was filtered by use of a membrane filter of 10μm in diameter. The filtrate was subjected to the same operation as inReference Example 1 to obtain electrolyte membrane AM8 having athickness of about 50 μm. When electrolyte membrane AM8 was subjected toan hot water dissolution resistance test, the mass loss ratio was 14.0mass %.

The same operation as in Reference Example 1 was repeated usingelectrolyte solution AS18 filtered to produce an anode catalyst layerand a cathode catalyst layer. MEA was produced by using the anodecatalyst layer, the cathode catalyst layer and electrolyte membrane AM8.

MEA was subjected to fuel cell evaluation as previously described. As aresult, current density after MEA was maintained at a cell temperatureof 80° C. and a saturated vapor pressure at 80° C. (corresponding to ahumidity of 100% RH) and a voltage of 0.7 V for 20 hours, was 0.01A/cm².

In Comparative Example 4, when a salt-type electrolyte was dissolved atan extremely high temperature, thermal decomposition took place andfluorine ion concentration increased and the molecular mass decreased.Because of this, the scattering intensity ratio A/B in the dynamic lightscattering particle-size measurement became less than 1.0×10⁻². As aresult, it was shown that the hot water dissolution resistance of theelectrolyte membrane and electrode catalyst layer prepared fromelectrolyte solution AS18 decreased and battery characteristicsignificantly decreased.

TABLE 2 Reference Reference Reference Reference Example ComparativeComparative Comparative Example 1 Example 2 Example 3 Example 4 5Example 2 Example 3 Example 4 Polymer MFR(g/10 min) 3.4 3.2 3.2 3.2 3.23.0 3.0 3.2 electrolyte Average particle 139 111 111 111 111 — — 111diameter (nm) Aspect ratio 1.0 1.0 1.0 1.0 1.0 — — 1.0 Equivalent mass(g/eq) 691 710 710 710 710 720 720 710 Solid-content 32 36.7 36.7 36.735 — — 36.7 concentration (%) Shape Emulsion Emulsion Emulsion EmulsionEmulsion Flake-like Flake-like Emulsion polymer polymer —SO₃X K Na Na NaNa H Na Na Water- Water/ethanol 100/0 100/0 91.7/8.3 100/0 100/0 100/0100/0 100/0 containing (mass % ratio) solvent Dissolution Fluorine basedpolymer 25 20 20 30 35 30 20 20 treatment electrolyte solid-contentcondition concentration (mass %) Temperature (° C.) 290 250 220 270 270240 290 360 Time (minutes) 60 120 240 60 7.5 120 240 60 Dissolutionsystem Batch Batch Batch Batch Continuous Batch Batch Batch systemsystem system system system system system system Electrolyte Scatteringintensity 1.2 1.2 1.1 0.7 1.8 1.0 100 1.0 × 10⁻³ solution ratio A/BFluorine ion concentration 123 101 89 100 79 706 145 1530 (ppm; based onthe mass of fluorine-based polymer electrolyte) Presence or absence ofAbsent Absent Absent Absent Absent Absent Present Present laserscattering peak Terminal-CF₂H amount 74 44 41 66 48 42 91 100 (%; basedon total number of terminals) Fe concentration (ppm; 7.7 5.2 3.0 4.30.12 42 15 29 based on the mass of fluorine-based polymer electrolyte)Evaluation Polymer electrolyte Absent Absent Absent Absent Absent AbsentPresent Present after remaining undissolved dissolution Mass loss ratioin 0.1 0.0 0.1 0.1 0.0 3.4 1.9 14.0 treatment hot water dissolutionresistance test (mass %) Fuel cell Current density (A/cm³) 0.44 0.450.45 0.45 0.46 0.04 0.09 0.01 evaluation

The present application is made based on Japanese Patent Application No.2013-139139 filed Jul. 2, 2013 with the Japan Patent Office and JapanesePatent Application No. 2014-058612 filed Mar. 20, 2014 with the JapanPatent Office, the contents of which are incorporated herein byreference.

INDUSTRIAL APPLICABILITY

The electrolyte solution obtained by the production method orcontinuously dissolving facility of the present invention has industrialapplicability as a material for an electrolyte membrane, a catalystlayer, an electrode catalyst layer, a membrane electrode assembly and afuel cell.

The invention claimed is:
 1. A method for producing an electrolytesolution, comprising: continuously supplying a polymer emulsion that isitself produced by emulsion polymerization and comprising a polymerelectrolyte and a solvent into a dissolution facility; continuouslydissolving the polymer electrolyte in the solvent by heating an interiorof the dissolution facility to obtain the electrolyte solution, whereinthe solvent contains 80 to 100 mass % of water and 0 to 20 mass % of analcohol; and continuously discharging the electrolyte solution out ofthe dissolution facility through a back pressure regulating valve whilemaintaining a pressurized condition within the dissolution facility;wherein a heating temperature of the interior of the dissolutionfacility is from 50° C. to 500° C.; and wherein the polymer electrolytecontains a fluorine-based polymer electrolyte.
 2. The method forproducing the electrolyte solution according to claim 1, wherein aheating temperature of the interior of the dissolution facility is from150° C. to 350° C.
 3. The method for producing the electrolyte solutionaccording to claim 1, wherein, in the continuously dissolving, a heatingtemperature of the interior of the dissolution facility is from 150° C.to 290° C.
 4. The method for producing the electrolyte solutionaccording to claim 1, wherein, in the continuously dissolving, apressure within the dissolution facility exceeds vapor pressure of thesolvent at the heating temperature of the dissolution facility.
 5. Themethod for producing the electrolyte solution according to claim 1,further comprising, after the continuously dissolving, cooling theelectrolyte solution while maintaining a pressure exceeding the vaporpressure of the solvent at the heating temperature of the interior ofthe dissolution facility.
 6. The method for producing the electrolytesolution according to claim 1, wherein the dissolution facility is atube.
 7. The method for producing the electrolyte solution according toclaim 1, wherein the fluorine-based polymer electrolyte has an averageparticle diameter of 10 nm or more and less than 500 nm, and thefluorine-based polymer electrolyte contains a —SO₃X group where X is analkali metal, an alkaline-earth metal or NR₁R₂R₃R₄ where R₁,R₂,R₃ and R₄are each independently an alkyl group having 1 to 3 carbon atoms orhydrogen.